I shamelessly stole the basic floorplan from somewhere I can't remember. I remember it was initially a wizard school, but I felt like the basic blueprint was better suited to a barracks-like thing, so... yeah.

The ground floor is on the bottom left and it's a common room.
The first floor is on the bottom right and it's an armoury and row of murderholes.
The second floor is on the top left and it's the guards' quarters.
The third and final floor is on the top right and it's the Captain's quarters.