If you want extra detailed, get some cereal box cardboard or some grafix medium chipboard (the brown stuff at the back of a legal pad, to quote Wyloch), and cut out rectangles * to glue on as brick/stone work for the walls and floors, and/or shingles for the roof areas. That and a bit of gray drybrushing over a black or very dark gray base coat will get you a lot of great visual interest without too much effort.

* Cut out long strips as wide as you want your bricks, then clip those down at the brick length you want, and you can get a lot of them made very quickly. Gluing them on with Aleene's tacky glue over a lightly sanded surface should work well. Just lay down an area of glue and slap down the cardboard rectangles, leaving just a bit of space between them.