My group has recently acquired an old guard tower as a base of operations. It's a fixer-upper, so I made a short document with some guidelines on ways to fix it up.
There is a really good "Stronghold Building" guide on DMsGuild: {WH} Fortresses, Temples, & Strongholds, rules for building and customizing player-owned structures! - Dungeon Masters Guild | Dungeon Masters Guild
But, it's large. So my goal with this document was for it to be short and to-the-point. It's basically like an equipment list, but for buildings.