The exterior doors of the houses are both metal and wood.  The wood doors usually have a metal gate over top on the outside of the house forming two barriers to breach.  The doors have two to three locking points.  Some doors are even barricaded from the inside to prevent entry.  There are generally two to three entrances to the house.  The entrances are the front, the kitchen, and the side or rear.

The interior doors are also made of metal and wood.  The differences between the interior and exterior doors are the strength and durability of the doors.  Interior doors only have one locking point and most of them can be kicked in.  All doors inside and outside of the house are usually locked and must be breached.

The layout of all the houses is generally the same.  Initial entry in the front door leads to a small room with two interior doors.  The two doors are the entrance to two adjacent open seating rooms. The size of the rooms varies according to the size of the house.  At the end of the sitting rooms are interior doors that open up into a central hallway. 

The central hallway is where all the first floor rooms lead and it contains the ladderwell to the second deck.  The second deck will contain more rooms and an exit to the middle roof top.  The middle roof top will have an exterior ladderwell leading up to the highest rooftop.

This is from Blackfive.