Picking up the idea of occupying a building, brings me to the point what to do to prevent people coming in. Let's say i'm involved in a firefight in a town and i want to hide, cover or simply take a crap in a building. Locking the door is the first step, but that doesn't keep anybody out if they really want to get in. Why not putting a metalwire across the threshold, so the evil bandits coming in will at least fall down (leg hurt or broken) and have to get up, which might give me the 3 more seconds i need to bandage myself.

More effective would be attaching a grenade to the wire, so the splint is pulled when someone stretches the wire.