I liked your idea a lot and, at least so far, it's been one of the more creative ones. Took me back to blockorz on coolmath lol.
I think it's fun but the enemies are slow and there wasn't much threat that they posed outside of their number. Maybe if there were some special enemies that went faster, made you slower, or maybe ate up some of the board until you've killed them would expand the idea even farther. Good job!