Agatha latched on to Tarvek's hand as soon as school ended for the day and tugged him around a corner. Tarvek considered protesting, mostly because Gil hadn't come out yet and was going to wonder where they were, but then Agatha started talking. "We're going to bake a cake," she said.

"We are?" Oh, yes, it was Gil's birthday tomorrow. "We're not going to bake one in a lab, are we? That's very unhygienic and I don't want to poison Gil."

Agatha giggled. "I don't want to poison Gil, either. We're going to Lilith's."

Lilith was their music teacher, as well as Judy from the Heterodyne Boys stories so it wasn't surprising she'd be closer to Agatha. Tarvek had never met her outside class. "Does she know?"

"Not yet. But I'm allowed to drop in if school's over for the day. I go there all the time."

"Do they know you're bringing a guest, this time?"

Agatha tugged him again. "Why would they mind you? You're never any trouble."

It was a strangely adult assessment from a girl half his age and a bit stinging. It wasn't as if he wanted to be any trouble — he generally tried very hard not to be, since his family didn't have much tolerance for trouble — but coming from Agatha… "Oh. Thank you."

"Anyway, it's important," she added.

Tarvek wasn't entirely sure about the relative importance of cake, at least in the minds of adults, but he gave in to Agatha's excitement and caught up with her so she could stop trying to tow him the whole way. She grinned at him and talked about cake flavours all the way to Lilith's door. Agatha knocked on it enthusiastically, the metal clattering under her fist, and soon the door was opened by Lilith. Agatha threw herself forward and was scooped up to hang contentedly around Lilith's neck, while Tarvek offered a hand. Lilith shifted Agatha to one arm in order to shake it. "I hope Agatha inviting me was all right," he said.

"I'm always glad to see Agatha's friends," said Lilith. "Why don't you come in?"

"We need to make a cake," said Agatha as the door closed behind them. "Urgently."

"It's Gil's birthday tomorrow," Tarvek explained, feeling that Agatha's statement was missing something. He looked around the room. There was floral wallpaper. Nice but inexpensive wooden furniture. Pictures on the wall. A cuckoo clock. It was possibly the sort of place books meant when they described something as 'cozy' or 'homey'. To someone raised in a palace it was completely new.

"Ah, so that's it," said Lilith, putting Agatha down. "You two go and wash your hands and I'll get some recipe books down."

They arrived back to find that Lilith had stacked a few recipe books on the table, brought in two sturdy looking stools so they could reach the counter, and produced two child sized aprons, leaving Tarvek mystified about why she'd had those.

"Are you going to want me to stay and help?" Lilith asked.

"We'll be fine," said Agatha, grabbing an apron. "We follow chemistry instructions all the time, it can't be any harder."

"We might need help getting the ingredients down," Tarvek said, looking at the cupboards some way above both their heads.

"Pick a recipe and I'll do that for you, then," said Lilith, smiling.

They picked a recipe for sponge cake that didn't look too hard — although Agatha was right, they followed instructions in chemistry often enough that they should have no trouble with this — and Lilith miraculously proved to have all the ingredients for it in her cupboards. "Try not to make too much of a mess," she said, ruffling Agatha's hair on the way out.

They set to work, Agatha softening the butter while Tarvek greased the baking tins. Agatha got a bit enthusiastic about creaming the ingredients together and wound up with sugar butter in her hair.

"I hope you never do that in a laboratory," he said.

"Things in laboratories aren't so hard to mix," Agatha retorted. "Now we have to add the eggs."

Her words proved to be prescient as well as descriptive. The recipe said to "beat well" after each bit of egg was added and the amount of beating it took to make even a little bit of egg disappear, as well as how much the cake mix seemed to be actively resisting, was phenomenal. "I'm tired of this," Agatha announced, shoving the bowl at Tarvek when it was his turn again, and jumping down from her stool to open a cutlery drawer. "I'm going to make something else to do it." Tarvek looked at the bowl, in which the egg was still not disappearing, and jumped down to join her.

They didn't have any motors or even any cogs or magnets or anything, so their design had to work by weight and counterweight, and involved the entire contents of the cutlery drawer, several rubber bands, and a lot of string. But it mixed the eggs in beautifully.

After that they had to fold the flour in, which they managed without needing to invent anything else, and then they could put the cake in the cake tins to cook. When it was done Agatha scooped some cake mix off the side of the mixing bowl and licked her finger. Tarvek copied her. "I didn't know cake tasted good raw," he said, surprised. Agatha grinned at him, and quickly scooped some mix off his side before he could get to it.

They had nearly scraped the bowl clean and were both laughing when Lilith came in and said "What on Earth?"

Tarvek looked up at the room and realised it was full of dangling cutlery on loops of string. "We can take that down," he said, quickly.

"No, no," said Lilith, equally quickly. "Leave it. I have to take a photograph of this to send to Barry."

"Is it that good?" asked Agatha, beaming.

"It's…remarkable," said Lilith. Tarvek realised suddenly that she was trying not to laugh and felt a rush of relief that that was the emotion she was suppressing behind carefully chosen words. "Now, why don't you clean up and I'll put dinner on?"

"I'm not sure we're staying," Tarvek began, thinking of Gil eating alone. Hopefully he would come back for dinner and not just stay vanished for the whole evening. Or maybe he should hope Gil would stay vanished, he knew Gil could steal from the kitchens. He told himself he was exaggerating, things weren't that bad for Gil anymore, even with both his champions away.

"We have to stay," said Agatha. "We haven't put the filling in yet, or decorated it."

Tarvek conceded and the two were on their way to the bathroom again when Agatha turned back and said anxiously, "Will you be able to cook with our egg beater there?"

"I've cooked in worse conditions," said Lilith. "Run along."

Adam arrived right before dinner and gave the kitchen a wondering look before offering them both a thumbs up. Tarvek was fairly sure he was trying not to laugh too. Dinner was delicious. Food on Castle Wulfenbach tended towards either the kind of unimaginative food made when there were a lot of people to serve at once, or the far too imaginative food that meant the older students had been allowed to cook. Agatha chattered through the whole meal, looking to Adam for a response as often as Lilith and apparently content with whatever he conveyed by expression or gestures. It was nice. Was this how people who weren't nobility lived? Although Agatha was nobility, and Adam and Lilith weren't her parents. They just liked her.

After dinner they washed up and then filled the cake with cream and jam and started making icing for it. "Is there any green food colouring?" Agatha asked.

Tarvek, who knew which poisonous substances were in which common food colourings and how much they could be increased to cover up a poisoning, said, "That has arsenic in it!" rather alarmed.

"Which is why we don't have it," said Lilith. "Sorry, sweetheart."

"White icing's fine," said Agatha. "Much better than poisoning Gil, anyway."

They did have some red food colouring, which was cochineal and not poisonous (Tarvek checked), to write "Happy Birthday, Gil Holzfäller" in piped icing. Then Agatha used it to draw a lobster underneath, which sort of made sense since Gil had been very taken with a book on crustaceans lately, but still didn't really seem like the kind of thing to put on a birthday cake.

"There," said Agatha, in satisfaction. She looked up at Lilith, "Can we pick it up tomorrow afternoon? It would be a bit difficult to hide it at school."

Tarvek thought he could hide a cake at school without too much difficulty, but Lilith had already agreed. "Now you'd better get back to the school," she said.

Agatha nodded and then threw herself at Lilith for another hug. "Thank you, I had a lovely time," she chirped.

"Thank you," Tarvek echoed.

Outside it felt as if the world had somehow become ten times bigger. Even though they were still in the metal corridors and not the echoing emptiness of the places still being built. The scale of Castle Wulfenbach felt off after coming out of a place that could have been a cottage. Tarvek took Agatha's hand and the two of them walked back to the school together, thinking of Gil's face tomorrow when he found out he had a cake.

They'd made him a cake. They'd made him a cake. Neither Gil's inability to remember his previous birthdays nor Agatha's indignant little squeak of "You're sharing cake with people who are mean to you?" did anything to dampen the gleeful discovery that his friends had actually made him a cake to celebrate. (And he did share cake with people who were mean to him. None of them were always horrible - well, almost none of them - and Madame Otilia was standing at the table like a guardian angel so they all were at least nice about it that day. He spent some time trying to explain to Agatha that celebrations were supposed to be for everybody while Tarvek tried to tell her it was a good way to placate them or make them feel obligated to be polite in return.)

Gil wanted to do something nice for her birthday just a few months later, but his ideas so far extended to building her a clockwork duck - he wasn't very happy with it yet, partly because he was trying to do it when she wasn't around, which didn't give him a lot of time - and making another cake. He had found out from Tarvek that they'd made his in Lilith Clay's kitchen, but neither boy was quite sure if it would be appropriate for them to ask her.

Then all his planning went to pieces when Agatha ran back in from speaking to her Uncle Barry (Gil barely hid the duck in time) and breathlessly invited them to Mechanicsburg. "The whole town's having a party!" she said gleefully. "Uncle Barry said I could ask you and he already checked with Baron Wulfenbach and Prince Aaronev."

"The whole town?" said Gil. He knew Agatha was more important than him, so it made sense her birthday would be too, but he didn't think many people had parties that big even in their school.

"Uncle Barry says Mechanicsburg is very attached to its Heterodynes," she said. "I'm pretty sure he means figuratively."

"He'd have to," said Tarvek, clearly trying not to laugh. "The Heterodyne Boys travelled a lot and he still does."

"Oh, yes, that's a good point." Agatha looked relieved, then bounced in place. "You'll come, won't you? Please come!"

"Of course," said Gil.

"Of course," Tarvek echoed, a little more formally. "I'd be delighted."

Agatha beamed at both of them. "Yay! I think we're actually inviting everybody, but I wanted you two with me."

"I guess it's a big enough party for everyone," said Gil, still a little awed by the idea of a party the size of a town. "I'm glad we'll be with you. It doesn't sound like you'll have time to even see a lot of your guests."

"Well, we can probably look at each other," Agatha said contemplatively. "Uncle Barry says the tourists mostly don't expect conversations, they come all the same if we're away, and I'm meeting the townspeople in stages when I'm visiting home."

"That's a lot of people to meet," said Gil, distracted. "Do you have to know everyone in your town, too?" he asked Tarvek.

Tarvek shook his head. "I don't think most people meet all their subjects. And we don't get tourists wanting to look at us."

"It's okay," Agatha assured him. "Mechanicsburg is kind of weird."

"I knew that," said Tarvek, and then looked guilty. "Sorry?"

"For what? I said it," Agatha pointed out.

"Nothing has to bite you this time, does it?" Gil asked, thinking of the last time Agatha had mentioned her family being weird.

"What?!" Tarvek looked rather alarmed.

"I don't think so," said Agatha. "Uncle Barry would have mentioned it."

"But what bit you last time?" Tarvek asked, not looking reassured.

"A big clank that's part of Castle Heterodyne's chapel," Agatha explained, gesturing with her arms to indicate an oval that Gil assumed to be the face. "To do a blood test. It hurt but not as bad as letting a cut wire snap back." She held up her hand to the light, inspecting it. "I'd show you, but I can't really find where it was now."

"That doesn't sound like it bit you very hard, then," said Tarvek, relaxing.

"No," said Agatha. "It just checks to make sure it's really got a Heterodyne. Um, and if somebody lies about that it kills them."

"Oh, that's..." Tarvek thought about it for a minute. "My family might kill someone lying about their bloodline, but the architecture wouldn't do it for them."

Gil snorted and flopped onto his stomach. "You guys take this stuff way too seriously."

Tarvek frowned a little. "Inheritance is serious. Of property and especially responsibility." And then, before Gil could say anything else, "That doesn't mean you're any less our friend."

"Okay, fine, I know you care about it," said Gil, shooting Tarvek a smile to show there were no hard feelings. It wasn't as if anyone had made Tarvek be friends with him. "And I guess everyone in Mechanicsburg cares about it since they're having a party because of it. But biting and killing people and everything is still a bit much."

Tarvek relaxed a little. "The biting is, uh, weirder than I realised," he said, glancing cautiously at Agatha to see if she minded how he was changing the subject. "What's wrong with a syringe?"

"I don't think they'd been invented yet," said Gil.

"I'm pretty sure her ancestors could have invented a syringe if they could invent a biting clank!"

"You can't expect one family to think of everything," Gil said, to tease them, and Agatha started giggling.

They really did pretty much invite everybody; the whole school was going to come down for the day, with Madame Otilia. Agatha was okay with this. She'd written to Uncle Barry after Gil's birthday, about the cake and what Gil and Tarvek had said. He'd sent back a long letter about courtesy and fairness and generosity and not holding grudges and remembering that there was something good about most people, even if they had been taught badly or made bad decisions for other reasons, which all came down to, he agreed with Gil.

He'd tucked her against his side and talked it all through with her again before the invitation, like she might not remember, but it was nice to be able to ask questions right away. Talking was still easier than writing.

Anyway, everybody was coming the next day and she was sure they'd have lots of fun. But the important part was she got to bring Gil and Tarvek with her.

The evening before her birthday, Tarvek left Andy with Theo (who promised to feed him, pet him, and shut him in Tarvek's room during the group visit so he didn't get into the kitchen and eat all the beans again), and they got into an airship with Uncle Barry and Adam and Lilith and Baron Wulfenbach to go down to Mechanicsburg. She caught the boys giving the grownups funny nervous looks and since Tarvek was being very formal she ended up writing him the question instead of asking out loud. "Why are you looking at them like that?"

Tarvek glanced at the paper and looked a little sheepish, then shook his head and started drawing things to disguise it. "They're more likely to notice if we're writing instead of talking," he muttered. "And I just hadn't thought about travelling with Baron Wulfenbach."

Agatha blinked. "But he's been Uncle Barry's friend forever."

"Well, yes," murmured Tarvek. "But he's...really different in the stories."

"Oh." Agatha hadn't thought about it, but maybe for people who were used to Baron Wulfenbach being in charge of things and stern and everything, the idea of him having fun seemed like it was as fictional as a lot of the other things in the stories. "Well, yeah, but they really are friends."

"I didn't think he'd want to go to a birthday party, either," Tarvek added, still ducked over his sketches. "Even with a friend."

"He looks like he's having a good time now, though," Gil said softly.

Agatha glanced up; the grown-ups were talking quietly among themselves, but Uncle Barry was gesturing enthusiastically about something and Baron Wulfenbach was grinning. "Uncle Barry says he and my father tried to get him a birthday cake once," she said. "Well, they did. Only they had the cook at Castle Heterodyne make it and the candles looked like little people carrying torches."

"Is your birthday cake going to have a mob on it, then?" Gil asked, sounding rather intrigued by the prospect.

"I don't know." Agatha thought about this. "I'm not sure it's the same cook anymore. But if you'd like there to be, I could ask. I bet they've still got candles like that somewhere. Uncle Barry!" She sat up and called out, while Gil started to protest. He probably thought it would be troublesome. "Am I gonna have a mob on my birthday cake?"

"What?" Uncle Barry sounded completely confused.

Baron Wulfenbach leaned back in his seat, one hand curled over his mouth and eyes very bright, and his shoulders shook. Agatha wasn't sure why he didn't just laugh out loud. "I think she means the candles," he said, after a moment. He sounded like he wanted to laugh, too.

"The-? Oh." Uncle Barry blinked. "I hadn't planned on it. Do you want one?"

Agatha tilted her head. "It might be interesting. Do they really walk?"

"There are a lot of variations on walking candles," said Uncle Barry. "I'm not sure a mob strikes quite the right festive note, but I can get you a different kind. If you're sure you don't mind footprints in the frosting."

Agatha looked over at Gil and Tarvek. "Would you mind footprints?"

Both of them shook their heads, apparently not sure about talking now the Baron was part of the conversation.

"Footprints are okay," Agatha said happily.

Uncle Barry came over to kiss her on the top of her head. "Then footprints you'll have. Uh, walking candles." He shook his head on the way back to his seat. "Thanks for the translation, Klaus."

"When did you even tell her that story?" Baron Wulfenbach asked, still sounding amused. "And why?"

"I don't remember how it came up..."

Agatha flopped back down on the floor, giggling and triumphant.

It was dark by the time they landed, and several happy Jägers surrounded the airship and walked them to the Castle, which waited until they were indoors to boom, "Welcome home, my lady!" and then muttered to Uncle Barry, "There. Fine. No public announcement."

"It's getting close to Agatha's bedtime," Uncle Barry said, amused. "I told you, if you want her to sleep here you've got to actually let her sleep."

"Yes, yes..."

"Hi, Castle!" Agatha bounced on her toes. Sleeping was the last thing on her mind at the moment. "You remember Gil and this is Tarvek!"

"Ah. Another potential consort?" it purred.

"I'd be honoured," Tarvek said. Then, when Agatha gave him a rather startled look - she was pretty sure they were all too young from what Uncle Barry had said, and she hadn't realised he knew what that meant, she and Gil hadn't until Uncle Barry told them - continued with, "I meant if you want to. When we're older. I mean..." And then buried his face in his hands while Gil snorfled at him.

"Thank you," Agatha said, reaching up to pat his arm. She hadn't meant to embarrass him, although the Castle might have, and that seemed like the best answer she could come up with. "Um, Uncle Barry! Is Miss DuLac coming?" That might distract the Castle, since Uncle Barry was old enough for a consort.

Uncle Barry cleared his throat. "Yes, she is."

"Shall I prepare a bedroom for her, or will she be sharing yours?" the Castle asked.

"A separate one, please, and if you were thinking about letting anything in there to encourage her to join me, don't."

"Don't be absurd," the Castle huffed. "Besides, the nyar-spiders would defend her."

"Oh, of course," said Uncle Barry, sounding very entertained. "How silly of me to forget."

Gil and Tarvek were going to stay in Agatha's bedroom, since it was big enough and they were only there for one night. In preparation for this the Castle had moved in a few extra four-poster beds, made of old, dark wood and decorated with skulls and gargoyles. Gil knelt on his to give the bedposts a dubious look, and then looked at the thick brocade hangings. "These seem more like tents than beds," he said.

"We can leave them tied back to sleep," Agatha said. She climbed onto hers and bounced on it. The mattresses here were squishier and bouncier than in Castle Wulfenbach, possibly because the grown-ups didn't want to encourage the students to bounce. "And this is great. I thought I'd have to come find you so we could explore!"

Tarvek looked dubious. "I know Castle Wulfenbach doesn't have many, but does this place have deathtraps?" He sounded like he was already pretty sure of the answer.

"Yes, lots," said the Castle.

"Yes," agreed Agatha, "but it can help us, too."

"Learning to navigate traps yourself is an important life skill," said the Castle.

"It was nicer than it pretended to be when it bit Agatha," said Gil. "And we stayed alone in a lab in it and it didn't do anything."

"I was not nice," protested the Castle, sounding sulky. "I was simply a little busy reintegrating. I will be sure to give you a proper tour this time. I have some lovely traps towards the Gate of Bones."

"Why are we exploring a homicidal building?" Tarvek asked.

"It won't kill us," Agatha said, eyeing the wall as sternly as she could. "And it's my house and I already told it I wanted to."

"...that really only raises another question," Tarvek muttered, but he didn't really sound like he was protesting anymore. "Can you at least get it to show us something other than death traps?"

"I am a death trap," said Castle Heterodyne with great dignity.

"But you've had people living in you for five hundred and eighty years," Agatha said. "Or five hundred eighty-one? You've got to have other interesting things in you."

"I do contain several laboratories, a collection of musical instruments, a lovely set of torture chambers, a nursery, kitchens, a seraglio and a storeroom of mostly deactivated clanks," said the Castle.

"Um, we can skip the torture chambers," Agatha said firmly. "The musical instruments sound nice. What's a seraglio?"

"A good place to keep consorts if you have enough of them," the Castle said.

Gil blinked and said, "Wait. I thought consorts were people you married? Don't you only do that once?"

"The Heterodynes have never seen any reason to limit themselves," the Castle said. It sighed. "Except for the previous generation. But I have high hopes of the one to come."

"Are you saying you want us both to marry Agatha?" Gil asked, and then ducked behind his fringe looking flustered.

"And are you going to greet all the boys who visit you that way?" Agatha added.

"Only the ones that seem to belong to you," said the Castle.

"Oookay." On the one hand, that sounded a little creepy. Gil and Tarvek were kind of hers, they were her friends anyway, but they weren't hers the same way Mechanicsburg was and all the other options she'd heard about were worrying. On the other hand it was hard to argue properly with someone when you weren't sure what they meant, and Castle Heterodyne might be nicer to them this way. "Maybe I should start warning them." She flopped down on the bed, propped on her elbows, and kicked her heels up. "What kind of deactivated clanks?"

"Mostly deactivated ones," said Gil, sounding a bit suspicious of that.

The Castle hummed. "Ones brought back from interesting battles."

War clanks. Agatha thought about that and decided regretfully that they were probably too interesting. "I don't think we have time for those in one night," she said sadly. Tarvek looked relieved.

"Maybe the musical instruments?" said Gil, looking like he wasn't particularly sorry to miss the war clanks either. "Or the labs. Gradok's was great."

"It was." Agatha perked up. "Hey, do you have any more kids' labs?"

"Several," said the Castle. "Is that where you'd like to go?"

"Yes, that sounds like fun." Agatha rolled off the bed, landing with a thump on her feet. "But Uncle Barry will probably come say good night so we'd better actually be in bed then."

They were, looking as innocent as possible, and then once her uncle had gone they waited a little while and slipped quietly out into the corridor.

Less than an hour later, Agatha was sitting on the edge of a rather complicated pit trap, drumming her heels against the wall beneath and telling herself as sternly as she could, which meant imagining Madame Otilia's voice from back when she'd been Von Pinn, that crying in frustration was not likely to help. The Castle hadn't let any of them actually get poked by the spikes, and it had let her find her way out, but it was blocking Gil and Tarvek. Agatha was tired and grubby and really mad at it, and there was brown on the tips of some of the spikes that might be poison and might be old blood, and once she got them out they were all going to have to have another bath.

"If you're going to be mean to my friends," she hissed at the Castle, "then I don't want to play with you."

"Being nice is outside my design parameters," said the Castle.

"Hmph." Agatha wasn't entirely sure if it was telling her the truth about that or not. She'd bet it could try. But apparently it didn't want to, any more than Madame Otilia wanted not to protect people, so that wasn't any help. She thumped one bare heel back against the wall again and tried something else. "That sounds very limiting."

"But I can be nasty in so many interesting ways."

"We were having fun up to now you know!" she whispered fiercely.

"I am still having fun," the Castle purred.

"I'm not," Agatha said sulkily. She peered down at Gil and Tarvek again. She didn't think she could get them out by herself, but... She sighed. A grown-up probably could, and the Castle wouldn't be willing to hurt her uncle either. Agatha stood up. "Okay. We don't know enough about traps yet, so I guess that's as far as we get. If you won't let them out I'll have to go wake up Uncle Barry."

There was a reluctant grinding noise. "Oh, very well," said the Castle.

Agatha stopped and looked down again. Some of the downward-slanted spikes had withdrawn and Gil was scrambling up with Tarvek just past him, giving the slots for the spikes a wary look. Agatha moved sideways so they'd have room to come up, grinning. "Thank you, Castle." It had done what she wanted, so this was probably one of those times when you were supposed to be polite to somebody even if they hadn't been nice before. "Are you both okay?"

"Yes," Gil said. "I prefer Castle Wulfenbach. I've always liked that architecture only makes things difficult accidentally," he added, sounding disgruntled.

"Sorry," said Agatha, since Castle Heterodyne was unlikely to apologize and it seemed like somebody ought to.

"I'm all right," said Tarvek. "Were you really going to get your uncle?"

Agatha made a face at him. "Instead of leaving you there all night? Of course."

The Castle let them reach the lab they were looking for without much trouble, after that. It turned out not to be as interesting as Gradok's, as this Heterodyne hadn't broken through so early or liked dragons so much, or maybe just hadn't left very many inventions there.

Agatha was yawning before very long and Tarvek suggested they'd better go back or she wouldn't be able to enjoy her birthday properly. Gil scooped her up and Agatha blinked and shoved at him a little. "I can walk!"

"Sorry." He put her down again. "You looked really tired."

"Not that tired." She hugged him before they started back to her room. It felt like a long way this time, even without any more traps. She fell asleep without her bath after all - the last thing she remembered was Gil and Tarvek picking her up and putting her on the bed.