When I first started creating Project Spark worlds I would try to build a mountain and then tunnel through it to create halls and rooms.  This really became a test of patients.  Once you have created a narrow space with ceilings it is hard to get your camera in the right place to know which direction your tunnels are heading, how close you are to the outside of your terrain and good luck placing props.  The solution is actually pretty simple.  Use the Add tool with the cube brush and make your corridors and rooms with an overhead camera as displayed above.  Once you have the first level laid out paint the room floors and wall, add props and light sources.  From there you can then use the Add tool again to create the next layer of your maze which creates you ceilings of the first level as you go.  I realize this isn’t anything amazing, but sometimes it is hard to see simple solutions.  Enjoy