I really like the concept of the game and it's quite fun to play, there are a couple issues I came across while playing, either bugs/cheese for the game.
1. you can place quad shooters strategically and just stand where they don't shoot, you could try adding an element of taking damage if the player doesn't move or something along those lines, maybe even adding an enemy that shoots at the player every once in a while to prevent being completely static
2. There are inconsistencies regarding placement, if you place a tower, you can place another tower 2 ticks(?) under it, however if you try to put something above it you need 3 ticks of space, at least with the quad shooters that is.
3. Adding the capability to remove pre-existing towers will help perfectionists not gauge their eyes out as well as allow experimenting with different strategies within the same game
4. An option to speed the game up, or add more difficult and tougher to beat enemies later on in the game would be great imo or just adding hitpoints to the normal enemies
other than these points I really like the game and find it enjoyable to play as long as I don't cheese it haha
edit: the audio also seems to be broken when not looking at the game, as in scrolling down or moving to another tab