One way is to go to Polygons on the menu and go to Triangulate. Clean up is a bit more involved. I suggest looking it over and experiment with each then undo and experiment w/the next setting in clean up. This way you see first hand what each does. Remember games use low poly because of frame rates. So try and get the polys as low as possible. Your castle looks great! I really does... not too shabby for a first model.