I like to use real castle and catacomb maps upon which to base my own dungeons and adventure environs. Doing so gives me some assurance that the environments are structurally sound and believable. Now, the castles from the above site are not real, but they are designed in such a way as they could be.
What I also like is when the layout appears to have been added to over time, giving the map a labyrinthine feel. This tends to give me an opportunity to have several areas that are more difficult to access. If the map has secret passages, staircases and the like, so much the better.