The dome is made from dental plaster like the rest of the piece. Its sectional - the dome can be removed to add a mini into the upper tower, the upper tower can be removed to add a mini in the lower tower... etc.
It didnt come out as well as I'd hoped, but its still cool for my first mold! hehe.
I've recently bought the Gothic Dungeon Builder #45, Gothic Church Mold #54 and the Floor Tiles Various Sizes Mold #201.
Although I've just completed the Gothic Church (still needs painting and a roof) I did find by using the Gothic Dungeon Builder for the basic bricks I needed to do many more castings. So if you were going to get started I would advise getting the Wizard's Tower Mold #50 or Basic Block Mold #40 to make your job easier.
Storage is the major problem with these things however, unless you have a lot of storage space its not worth it.