Overall this concept has immediate appeal and the player knows what the goal is right away and how to get better at it, very well done. Upgrades in this game are satisfying to obtain. At least the ones where you get to place more towers anyways. They have a very clear effect and identity and really compels you forward. The radius ones are a bit less enticing because they're small, which I assume you wanted to be careful with. Once the walls come into play, you reach end game fairly quickly from there.
Getting walls set up reminds me of Jezzball, where you're sort of corralling the enemies and locking them into certain areas. It feels very good to pull this off. With some different paths and enemies this could turn into something even cooler :)