Creating dungeons is quite a bit different than creating a town. Thankfully there is more than one way to do so. The game allows you to pick the walls and floor and it will randomly generate dungeon which obviously makes it quite a bit easier. You simply ass a few things and it done, which is what I have done in the above picture. The other option, and the better option for any serious game maker, is to design the map from scratch. There are various tiles sets you an use to do this from floors and walls to statues and rugs.
Another thing to remember is where in the game you a placing it. A huge sprawling dungeon at the very beginning probably is not in the best interest of whoever is going to be playing your game, but a single room also would be pretty much boring. Also keep in mind you do have to create the monster for your dungeon and decide how strong you need/want them to be. Just remember to have fun, and may the gaming gods bring you glory.