More procedural map generation. A problem to solve is locked door/key placement.
Some rooms are connected by hallways, and some are connected by locked doors.
If a map is to be self-contained, then there must be an accessible
key for each door that's in the path of the exit. But it's more
interesting to have extra keys lying around and some locked map rooms that
aren't on the path to the exit, so the player can decide whether
to open a room on the current map or save a key for a later map.