I recommend using the advanced search feature to look for "dungeon gimp" in the Tutorials/How-To forum. Or just use this link:
http://www.cartographersguild.com/se...archid=3223980

Although most will be targeted at underground environments, the techniques are easily transferable to above-ground buildings and castles. There was one by… Duvik? I think? That I found useful long ago. I'm sure there have been many advancements in the years since then, both in the software and the techniques, though.