Actually quite interesting. If I may suggest, you can play with turning towers on and off, helps push the agenda of letting some enemies leak. I feel like the system needs some rebalancing between the prices and how much health you can 'spend'. Actually, it would be interesting for a tower defense where you literally pay with your own health in the first place. Maybe offer lifegain if you beat a round perfectly?
Sorry for the huge suggestions, its really interesting! I love the start screen and I love what you were going for. Great game!