I agree with ChaosChild. A little rubble will go a long way. A couple of small pieces of broken balsa, some gravel or small bits of shale, maybe a couple of small chunks of plaster and a loose weapon or two. You don't want so much that the minis are going to tip over. Also, the tile floor looks too clean. A dark brown wash with some ground up pastel over that would do a lot for the piece.