This is the start to a solid TD game! It seems like a nice base to add more enemies and weapons.
One thing that is a potential low-hanging fruit is enemy rewards. It took too many rounds to get enough money to buy the second turret. More payouts/harder waves might improve the pace of the game.
Things look nice, are easy to understand, and the game feels stable and complete--great job!
I look forward to trying again with a future update!