Interesting idea but I found the controls hard to get used to. Im so used to left click being the way to select towers in this kind of game. Some more feedback like sound effects for selecting a tower, killing an enemy, etc would make the game feel a lot more satisfying and easier to understand. Controlling multiple towers at once is satisfying and I could see that mechanic being expanded further with more towers. Good job!
Side note: I found a bug in level 1 where as soon as I kill the first enemy that spawns I win, even though more enemies spawn after the win screen