Great idea! It is very promising, but I think there are many things can be optimazied.
1. Audio: I think my headphones are broken at beginning, but It is real no aduio in this game, I think some simple sound effect are very helpful, It can improve game feel very much! It will not spend too much time.
2. Game Balance: It is too easy, increase the number of enemies and more poweful enemies should be interesting. When I overcome powerful enemies, more fun I will get in this game.
3. UI: You can hide the Tower Card when the enemies attacking, so that the card will not be overlay the buttom of board and players will not try to create tower in this time.
Overall, good job!