Good concept, and I like your take on the theme! It rewards risky gameplay. I also really like how you're able to move your towers around, so you can constantly reposition depending on what paths the enemies take. I would remove the restock mechanic entirely though, as it makes it very hard to upgrade your towers because you have to constantly spend money to restock ammo. It's also difficult to tell when your tower is out of ammo or not, so an indicator for that would be useful.