A nice, creative idea, fells quite polished too! :)
Its a bit hard though, the enemies have too much health (in my case most of them just ran through everything the towers shot at them) and the mechanic to loose money when you collide with an enemy just makes it even harder (i guess the stun time you get is already punishment enough, as you loose time in repositioning your towers)
If you want to continue on the idea you could make different kind of dice (maybe even customizable between levels) for example, make the player choose what kind of tower is on wich side, so you can have more then 6 tower types in your game but only use the 6 per level (maybe you could a different dices too like a D4)