This is absolutely fantastic. The rest of the comments are raving, and I'm all aboard the hype train for this one. Easily one of the best submissions for this game jam. Here's my more detailed review:
Design [5/5]: I was a bit concerned that this wasn't going to really be a tower defense game. Thankfully, my concerns vanished as soon as I started to play. This is very much a tower defense game, and a well-thought-out one at that. The varied enemy types are all balanced well with your character's move speed, the tower's range, and your attack power (all of which are upgradable, but why would you when you can have a cool hat?). As far as balance goes, the game was challenging, but completely fair.
Most of the time, when you're playing a tower defense game, you aren't just sitting around idly. You're usually looking through tech trees and upgrade menus and building more towers. I was worried that, since you only have one tower, you weren't going to have anything to do while the tower takes care of the enemies on one side of the battlefield. Thankfully, there's much more to do than just move the tower around thanks to the brilliant upgrade system. Rather than accumulating currency by picking off enemies like in most tower defense games, you accumulate currency by collecting crystals that appear in random locations in the battlefield. It keeps you occupied while the tower fires away, meaning there's almost never a reason not to be moving, but it does even more than that. Since the crystals are the only way to upgrade the tower, you have lots of motivation for collecting them, so you have to strike a balance between moving your tower to the optimal location and hunting down crystals. There is never a dull moment with this game because of that system.
Another thing I love about the upgrade system is that it's only as serious as players want it to be, thanks to to the "cool hat" upgrade option.
The boss battle is a brilliant addition to the game as well. It forces you to juggle three things at once: fight off the waves of enemies, collect crystals, and hit the boss. The game had me fully engaged from start to finish. What a brilliant design!
Adherence to the Theme [5/5]: This is a wonderful interpretation of the theme, and I think you guys turned an odd-sounding restriction (only one tower) into a brilliant little game.
Originality [5/5]: The game is totally unique as far as I'm concerned. It's a breath of fresh air in the tower defense genre. Well done!
I seriously can't applaud you guys enough. This is a contender for my favorite game of the entire jam. I don't know whether or not you two have future plans for development on this game, but I would support you morally and financially if you chose to do so. Great job! Bravo!