Haha that difficulty curve sure SPIKED right there. I had a nice little ring of the new weapons, and suddenly a horde of 40 enemies just bum rushed the crown. Oh well, it was fun up till that!
It was a really interesting game, kind of mix between top-down shooter and TD. I wished that I was able to switch weapons without taking my hand off of WASD, but I'm not sure how you would solve that problem, using the mouse to direct fire. Other than that, the only other issue I had was that some gems would spawn within places I couldn't reach.
I thought the level setting was well designed, good use of the tiles for that. The start menu was a nice touch, but I don't think your other menus got as much attention. Still, for a prototype, it's better to have the functionality in there than not at all! And like others have said, the tutorial box staying visible did feel a little distracting.