the cube is just an addition, there is nothing inside it... or more to the point, there isnt nothing inside it...
nehu, u need to either make a hollow cube, or subtract a smaller cube from inside it to make it into a room cube... then subtract away one of the sides so its like... erm, a box on its side, then add the door(s) to the subtracted side... and if the terrain disapears when u do a subtraction, dun worry, just build all.