If Merlin thought about it, he could maybe remember a time when he wasn't the prince of Camelot.
He had a mother. He remembered quiet, warm, and soft. And brown. He remembered brown.
And there was the journey to Camelot. It was fuzzy and hazy, but he was with Gaius then without so much as an explanation for why was he was suddenly living in Camelot at the age of four. And he'd never bothered to ask why if his mother couldn't be bothered with raising him herself.
Then there was Arthur.
Merlin had lots of memories of Arthur.
The first though...
Merlin blinked at the boy in the middle of a pack of squirming puppies. "I was just...looking."
"Well no one said you could," the boy said with a fierce frown.
He unconsciously lifted a hand to his own cheek where the boy's mark was. "I heard barking..."
The boy rolled his eyes. "Are you simple? This is the dog den. Of course there was barking."
"Are you okay?"
The boy stood, eyes flashing. "You're not supposed to be here. Who are you?"
"I'm Merlin!"
"What kind of name is that?"
"Well. What's your name then?" Merlin retorted.
"Arthur."
"That's the prince's name."
"Of course. Because I am the prince, idiot."
"You're certainly not acting like it," Merlin snorted, crossing his arms. "You're more of a prat, aren't you?"
"I wouldn't know, Merlin..." The boy said pompously. "But it seems that you, Merlin, are idiot enough to thing that you can get away with speaking to me like that."
Merlin snorted. "Just because you're the prince, doesn't mean that you can be a prat."
"You said that already. Is your vocabulary that limited?"
"I—" Merlin felt his face flush and when Gaius walked in on them later, he found the two boys on the floor, entwined, rolling one over the other, Arthur mostly pinning him to the floor. Uther later laughed at the myriad of scratches and bruises the boys gave one another. (Merlin had been taller, giving him the advantage, but Arthur had had the training. Giving him the bigger advantage.)
His next memory is of coaxing out the story that Uther had slapped him for misbehaving while in court.
The next memory after that is special. Merlin was seven. Arthur was maybe six. He found Arthur sulking up on the ramparts, knees curled up to his chest. A dog wriggled in his lap while Arthur stubbornly held on, expression broadcasting that he was comforting the pup, though it was clearly the other way around.
"Why do you always follow me?" the little prince asked, voice sullen and choked with previously shed tears.
"I..." Merlin shrugged a thin shoulder. "I saw you run off, and I wanted to see that you were okay."
"What business is it of yours?" he scoffed but dropped his eyes to the side.
"I can cheer you up!"
"Oh really? And what could an idiot like you do?"
"Magic! I'm magic," Merlin confessed with a grin.
"Uh-huh. And you're smart enough to go around telling the prince of Camelot that," Arthur muttered, brow furrowing.
Then Merlin squatted in front of him and spread his fingers, coloured lights dancing between them to form the images of men in a grand battle. He grinned at Arthur's sudden awed expression. However, the pup was quickly interested and Merlin found himself toppling backwards, dog slobber on his face before they were both running pell mell down the stairs after the errant dog.
Collapsing behind the throne, they laughed, breathless. "That was pretty great," Arthur admitted, holding the dog to his chest.
Merlin grinned.
The next minutes he has trouble remembering properly because it got very confusing. The doors creaked open and then there were voices muttering about finding the prince and Arthur pulled him tight to him, shushing him with a finger to his lips. Unfortunately, that meant that the pup got squished between Arthur and himself and yelped. The strange men called out, "Who's there?"
This was where it got mostly confusing—Arthur tugging him down and making to rise, Merlin panicking, the dog getting loose, the men coming closer, Merlin shoved Arthur down and stepped, trembling, from behind the throne. The men stared at him hungrily, mumbling about the prince.
Merlin held Arthur in place with magic, taking a step towards the strange men, telling them in as confident a voice as he could manage, "You're not supposed to be in here. Get out."
Somehow this, predictably, did not end well, Merlin squirming in the arms of the strange men, panicking and yelling his fool head off.
Which brought the guards.
And Uther.
Who immediately yanked Merlin-who-looked-like-Arthur into his arms.
"You will never touch my son again! Do you hear me!"
Even Merlin quaked in the king's arms.
"Take them to the dungeon. Let them rot there," Uther hissed. "And you," he transferred his attentions to Merlin. "What are you doing in the throne room, Arthur. I specifically told you to stay in your chambers this afternoon!"
"Wait! Father!"
"Arthur?" Uther looked down at the boy in his arms and then the proud child standing by the side of the throne. "What... What is going on here?"
"I'm Arthur," Arthur said.
"And you?" Uther's gaze bored into Merlin.
Merlin squeaked.
"That's Merlin, father."
"You lie."
Merlin changed back immediately. And then stumbled to the floor when Uther flung him out of his arms.
"Sire! I just heard—" Gaius burst into the room just in time to see Merlin change from Arthur back to himself. "I—my—Sire?"
Uther sent a sharp glance in the physician's direction and then turned his piercing gaze on Merlin. "You. What was that?"
"It...I...it was just... I didn't—I wanted..."
"Enough," Uther said impatiently with a wave of his hand. "Out with it."
"He was doing it to save me," Arthur said proudly, even though Merlin could see his fingers twitching nervously.
"Arthur, be quiet."
"It was—"
"Hush, Gaius. Merlin was it? Explain. And be quick about it."
"M-magic," Merlin mouthed, the words barely more than air passing between his lips.
"I figured that." Uther's eyes narrowed suspiciously.
Merlin could feel his life hang in the tip of a scales. Opening his eyes wide in an expression that had always gained him favour in the past, he held his breath and waited.
"Sire, he's just a boy. He doesn't—"
"Gaius! Hold your tongue," Uther snapped, lips compressing as he inspected Merlin, looking into him and saw his soul. "I'm thinking."
"Father..." Uther cut him off with a wave.
"You might be useful," Uther said finally. He straightened himself and gripped his hands behind his back.
"Usef-ful?" Merlin blinked.
"Yes. I have decided. You are to be Arthur's double."
"Double?" Merlin and Arthur echoed simultaneously.
"Milord," Gaius began.
"My mind is made up, Gaius. Make sure the boy is caught up on all of Arthur's current learning, and see that he is trained in the way of the sword. I want them equals by the end of the month. Can you see to it?"
"I...Sire..."
"Gaius."
"Yes, Sire?"
"You do understand what I am asking, do you not?"
"I—yes, Sire. I will do as you have asked," Gaius whispered.
"And you, boy. You will be Arthur's double. You will swap places with him when I deem necessary, and you will learn his studies so that you may pass as my son. Can you do...that...enchantment again to appear as my son?"
Merlin nodded.
"Do it. Right now."
Merlin squeezed his eyes shut, shimmered, and turned into Arthur.
"Impressive," the king murmured. "That will do. Change back. Right now. Go with Gaius. I want you to begin today."
And that was the beginning of his education.