This is an awesome game - the best I've seen so far! The concept perfectly fits the 'only one' theme, and you managed to pull off a one-tower tower defense game, something I thought wasn't possible. The collecting gems mechanic gives the player something to do whilst the tower is at work, and rewards the player for setting up a tower in a way that requires less movement. The art is really good and the game is overall very polished. The only thing I would say is that the enemies should be a little more varied - either different types of enemies in each wave, or make them move in different directions. There were a few stages where once I had put my tower in place and it was dealing with the flow of enemies fine, I knew that I was going to beat that wave with no more effort - having the possibility of a wave suddenly changing form would keep the player on their toes a little more.
Thanks a lot for the detailed feedback, it means a lot to us! And thanks for being so detailed on the design point of view, my intentions were the same that you described, which makes me very happy hahaha! About the waves, I completely agree! Since I'm not super good at prioritizing stuff, we've added the little hat instead of doing more enemies, so we run out of time at the end, my fault! We're thinking about adding a little more content to the game, so more enemies and better wave distribution might be one thing to add!