Oh it's not meant to stack one on the other. You lay out the dungeon as you go, and when you go to another level you pull the pieces aside and start the next one.
And I'm making mine with half-height walls. Just so you can see where the walls are. That way players can still look over the walls and see the figures without standing up.
There's a lot of potential for making buildings, bridges, castle walls, more complex dungeon tiles, etc. But my primary concern is to not block line of sight for the players.