Dungeon Feature - Doors
Dungeon Features - Doors
Unless you have the ability to walk through walls, you are going to need an opening of some kind. Doors would be the best way.
Although designed with dungeons in mind, the Dungeon Feature series can be used on any appropriate fantasy or medieval style structure.
This generator, or series of charts, is designed to help flesh out any needed details you require. Use all of them or just one, they are entirely optional.