Regarding defense: You could try a gatehouse of sorts. The building has a door at the front and another one at the back. In the building, facing the rear entrance, is a security desk where the NPC sits. Then you can have search lights mounted onto the wall above the rear entrance, turrets on the roof, and maybe some of these dome turrets hanging from the ceiling inside.
Maybe like:
Level 1: just the apartment guard with his desk and some search lights
Level 2: one turret on the roof or on the second floor. Maybe poking out of a window.
Level 3: two turrets on the roof or second floor, and also a dome turret on the ceiling in the lobby.
I think I posted something like that already, but still.