Dungeons are traditionally ancient underground structures and are subject to decay from water and ground shifts. Over time and lacking the methods of modern building techniques the walls would warp and even crumble in some areas. When I build my dungeons I never try to keep the walls straight, as warped walls and the occasional block sticking out adds to the character of the environment. The more decayed you can make your dungeon look the better the model will convey the dark dingy feel your aiming for. One technique I use is stacking, to do this build a double wall, or wall and a-half. Leave gaps in some areas of the outer wall and chisel away in others, to revel the walls second layer underneath. This not only adds decay effects but also adds depth to the model. Painting techniques such as washes and dry brushing will also help give your model that old decaying look. Colors should also be kept dark and gloomy, black, gray and brown are all moody colors. Green should of course be used for moss or slime. Red is another great color to use as it conveys a sense of danger and aggression. Some blood on the walls, floor or bodies is a nice touch but use the philosophy of less is more. The use of bright colors should be limited, using them in treasure rooms, wall decorations, furniture, and such. In this way those little details, or center pieces you want to draw attention to will really stick out. Scale is another issue that needs to be addressed, in real life trees and buildings tower over people. If you want maximum impact from your models they will tower over your miniatures to. Bigger really is better. Of course your limited by space, and the time you can spend making models, but I try to build as big as my space and time permits. A few nice large models on a gamming table will look better than lots of small ones. Most of my dungeon section are 3 inches high and one foot square. While a bigger piece would be better this allows them to be stacked for storage. I may try to build a larger section as the dungeons center piece. To texture the floors I often use fine play sand mixed with light brown paint. When dry add a chestnut ink wash. Let dry, then dry brush with an off white or light brown. Last fill your dungeon with as many details as your time, budget and imagination allow. Torches, wall tapestries, crumbling stones, bones, skulls, writing on the walls, furniture, spiders, rats, gates, statues, and dead bodies keep things interesting and will make your model come alive.