I've come across some issues in my short play
- several crossbowman will all target the first unit if they can. the first shot hits, next shots miss. I think auto-targeting a different unit is better. Can also allow targeting[first | last| strongest] like in bloons TD
- targeting a unit on top of a tower(for removal) is tricky, so is placing it. very tight margins compared to empty grid spaces
- I placed a spike trap atthe bottom of the ramp at level 1 but enemy pathfinding simply skirted it at the tile edge and it was ineffective.
- once I have been defeated, auto-restart the page in 2 seconds or pop up an alert letting me restart, the units keep on going currently.
The graphics are pretty nice and the game logic seems to work pretty well, so good job there.