Great concept! I like the dungeon building to continue, though there're times the map starts without the possibility to place any tile. You can add more to it easily and could be really good.
I recommend that upon reaching the stairs it just starts another "level", maybe changing the start point to show it's underground. You could also start adding boss tiles or awesome item tiles or trap tiles. It all comes down to if you have other options to continue or should risk shooting yourself on the foot hoping for a good tile next.