Well, I prefer a medium (sometimes small) POI - since with big ones they can spawn inside your base at times; which can lead to disaster. Multiple entry exits for you - just in case. If it has multiple levels you can use the 'floor collapse' as a bonus defense, dropping it onto the zeds below lol I like the big firehouse at times. I'll use the garage part to fight in, having the area from the 2nd floor living space to shoot from, that gives cover from vultures and sometimes cops. Either that or use the steel things at the rear to fight from. In that living area I'll allow the zeds to get into the first floor and I'll have holes in the floor of the 2nd floor by those doors on the first floor to shoot down at them. But with almost everything there being concrete that takes a long time to make those murder holes.
Hmmm... You actually got the main part... So if the POI doesn't meet that requirement its out. If you look at a great deal of the POI's that excludes a tonne of them! So a few things I look for are entry points for zeds, or ways they can seemingly get into the building - but having the entire first floor covered with spikes and such - having some murder holes to shoot down at them. OH! Upgrade ability of the walls or structure in general, since sometimes you may not have enough time to make your first horde night bunker. In this case I'll see if it's possible to be able to use multiple bunkers/POI's - having a makeshift walkway from one to the next. Making it available via dropping frames/rebar so that they don't see it as a viable way to get to me.
On the outside walls I'll put up frames/rebar so I can walk on them and ensure I can shoot them if they're pounding on the support walls.
Sometimes I'll put up a pole on the outside with access that i'll have to drop from the building - so I can run out there to draw them out of the building if it starts to become unstable.
As I alluded to before you can actually make that first upper floor a weapon against them, but generally I use that as a last resort. Some of the POI's you can do this multiple times, but those are generally too big and you definitely don't get exp for killing them this way. Besides - Big POI's can have you running more than your killing zeds.
I also like those smashed up 'business' buildings - the ones with missing floors, sections of walls, etc. That one that has a partial 2nd floor with a decent amount of walls and in one corner about 3 or 4 floors. I'll build up the 2nd floor and make a make-shift roof, lining the 2nd floor (again with murder holes) and the outside walls with frames/rebar.
Since many POI's aren't that stable I'll generally put stabilizing blocks inside the structure, I used to double line the walls but that's a lot of work.
