Nurmengard, last refuge

On 14th October 1981, Lily Potter née Evans has an idea that would ensure their safety while also allowing James and the Longbottoms to go on missions again. The fic follows their next few weeks, from settling in Nurmengard to a full-on siege of the castle on Halloween. Warnings: the characters aren't safe from whatever happens in the original novels, including Unforgivable curses