A very cool tower defense game! I sort of wish there was more to do, but that does mean whats there makes me want to play more! I think the difficulty stops increasing after a certain level, which means once you've set up correctly you can basically go on forever. maybe having enemies slowly increase in hp wave after wave? and maybe a way to gain back people and upgrade more than once, so that the game can keep going for longer!
Anyways, great job!