I enjoyed it! Needed a few tries to figure out what upgrades were important to get before certain waves but that's ok since the rounds are so fast. Might need to give the player a little foresight if you're planning on making the rounds longer.
For instance, dying to the first dragon because you don't have long range arrows is fine since it doesn't take too long to get there. But it would suck if there are more gotcha's like that 10 minutes into the game.