John Stall's walls, mentioned by casamerica are a major life saver. Each is a character that includes a wall as a prop. That wall can be made to disappear with the heirarchy menu and another wall with a doorway can be used in it's stead. I have also "cut" a doorway in a wall with a transparency and used JOhn's walls/windows. In one case, I replaced the "core" that comes with the set (click on heirarchy) and parented another door I liked to the core (now invisible but there) and it worked like a champ.

Gordon