OK, I think I'm going to use this in my next map, so I'll reveal it to all now.

The basic idea is to have multi story apartment blocks. First you have your city laid out, with your apartment block somewhere.Have a location or whatever for entering it.When a unit hits that location, it is teleported to an area off the normal playing area, with walls/cliffs/whatever surrounding it to make it look building shaped.
Inside that area, you have a door for exiting, plus indoor stuff like beds, TV's etc. You also have terrain/a unit for the staircase. When the unit goes into that location, you teleport him to another area reserved for this purpose, then remove all the beds TVs etc.Using preplaced locations(probably laid in a grid all over the floor) you then use triggers to place more beds TVs couches etc. at different locations.
When the player gets to the top story, you can teleport him to the city at the top of the building he entered. Obviously this gets quite complicated the more stories/apartments you have, but it should still be usable.

Hope this is useful to someone.

Nothing new, but, upon entering the building the player should be in a large staircase, which would allow traveling between stairs.

Do you mean when you first enter the building, you are standing on a staircase?

Yes, just like that.

But if you have it thhat when you first enter you are standing just inside the door, and you have to walk to the staircvase, it makes it much moere realistic.

Either he is saying you enter the building by going UP a staircase or that when you enter, you are already ontop of a staircase (Side Door Entrance!?)

But yea, walking to a staircase is more realistic. I've actually done this already, if only I could find that map...
Note to People: Clear out your Downloads Folder!

This has already been done extensively.

Locked.