Got a solid start there. The fundamentals of a tower defense game are there you just need to dial it all in. Tower defense is about the balance between your resources and the difficulty of the level. You only had one wave and I was never worried about resources. There is a moment in the beginning where you only have one tower and are worried you might not get enough money before the enemies make it past that tower but you just make it and place the 2nd tower, beating back the line and getting a breather. That is what you are chasing as your peak tension. You never got back there for me.
All its going to take a time and balance of the enemies and cash they give. More tower choice helps as well.
Thanks for the game