Upgrading wood stud walls-
Tier 1: this is the wood stud frame. You can place this and never upgrade it if you don't want. But, players can walk through the center of it. These are pretty easy to mass produce as you all you need is a hatchet.
Tier 2: this upgrade places corrugated metal on the side of the wall that you were facing when you placed the wall. You can stop upgrading the wall at this point if you don't want a normal doorway in it.
Tier 3: upgrading to this point cuts a doorway into your wall. This tier does not give you a door yet, just the passage way.
Tier 4: this gives you the door but not a lockable door yet.
Tier 5: this gives you a lock on the door.