Research into the warding had to wait when they returned home. A large owl was waiting with an official demand for Hermione to hand over Grindelwald which meant that the ICW could attack at any time.
She stood with Gellert at the gates as he activated dormant protective enchantments that she'd never even heard of. The elder wand was familiar in his hands as he layered silvery magic across the portcullis. A flick of his wand closed the massive wooden doors with the groan of unused hinges, the boomed shut, echoing ominously up the hall. Mechanisms grated, slamming massive iron bracing bars across the join and thoroughly locking them. Yet more spells reinforced this, then spectres dressed in world war II uniforms whispered into ranks in the hallway.
They shut a final set of doors, these ones locking as well, then Hermione sent her owl out to the parties needed to put her plan into action. Then she stunned the unfortunate bird that had delivered the message from the ICW, hoping that it would give her even more time to prepare the castle. The elves were given orders to begin stockpiling food and sent to sellers and producers across the continent to find everything they'd need to keep everyone fed for several months.
Ever proactive, the residents had sent up a list of the potions they had brewed which she was happy to pass over to Grindelwald as the returned master of the house. He grumbled something about it being her war but looked over the list anyway as she sent a message to Ginny that a team needed to help the elves cast stasis charms on food. She received her positive reply as she was pulling out all the books that they'd taken from Slytherin's personal collection.
They easily quartered the books that they would need to look through by putting aside all the ones by wizards other than Slytherin. He had developed the technique after all.
Then it came down to looking through the notebooks and diaries that he had written to find the section on warding. It was slow work, the books had all been written before the Norman invasion and the English used was hardly recognisable. She begged Harry to stay, and he remained reluctant until he found a section on possible improvements to the broomstick enchantments on the school brooms.
She fought her way through several volumes that seemed to contain hundreds of different ideas, all jotted randomly throughout the book. Opposite her, Gellert mumbled under his breath in German as he struggled through this archaic version of his second language.
His perseverance paid off though, as they were finishing dinner he made a triumphant noise as he opened his second book of the day and passed it over to Harry and Hermione. The first page was a rough map of the ley lines across Scotland. The second was filled with a tight script which she dreaded trying to trawl through but concluded it was cruel to make Gellert do it.
It turned out to be horrifically complex, most of Slytherin's notes devoted to controlling the cataclysmic power of stronger ley lines. He'd managed to perform the spell early on in his research but struggled with anything more than the faintest line. Grindelwald assured her that they were on the intersection of two.
Magic itself had been very different in that time, most of Salazar's work seemed to assume wandless magic was the norm with wands being unstable. He lamented several times over how his experiments had failed because his wand simply didn't want to work. He'd also mentioned his staff several times, which in the 900's seemed to be considered a much more reliable form of channelling magic.
Fortunately, Hermione knew that Durmstrang students learned to use a staff from when she had dated Krum. The quiddich player had complained about it once, calling it outdated and useless. Gellert was happy to take over that aspect, confident that if they converted the staff sections to wand it would go a long way towards helping to control the flow of magic.
Hermione stayed up until the early hours of the morning, working through the process of the wards and Slytherin's experiments. He seemed to have structured the book almost like a diary and she couldn't help but get a sense of his character. He was a cold man but loved his school and shared powerful friendships with his fellow founders. He thirsted for knowledge, revelling in the unexplored boundaries of magic and dabbling equally in both light and dark magic. It was, Hermione supposed, what made him such a powerful wizard; Voldemort had pushed only the boundaries of dark magic, convinced that anything light was weak and useless. He'd limited himself and as Gellert had once said, he'd missed subtleties that he would have picked up on with a broader knowledge base. What made both Slytherin and Grindelwald his superiors is that they recognised the power of light magic without the constraints to just the light that had limited Ravenclaw and Dumbledore.
An interesting discovery was that Slytherin's hatred of muggles had been interpreted almost completely wrong. He didn't hate them, he was terrified of them. He made passing mentions of the witch hunts and burnings, lamenting over a child that he couldn't save. His only grievance with muggle borns was that they were often the ones found by the witch hunters and he feared that they would spill the secret of the two Lords and Ladies of Mòrthlach under the tortures reserved for those condemned as witches. It was this that spurred him to create a version of the muggle repelling charm that would keep the muggles away forever.
She also discovered that his moral compass relied almost solely on the opinions of his peers, he would often write down several different options for experiments and rank them in order of which Godric would approve of most. Hermione wondered if, like the many other similarities they shared, Grindelwald thought in the same way. She knew enough of history to know that he'd had issues when he was at Durmstrang, she knew Dumbledore had seen the version of Grindelwald that she did; there was always the potential, he had an aura beneath his carefully controlled expression, a certain volatility about him that made it seem like he was only just held back from unspeakable acts. When his friendship with Dumbledore had broken down, whatever restraint had been in place disappeared. She felt queasy as she realised just the position she held, it was more responsibility than Deputy Minister for Magic. She was Grindelwald's Godric.