It was a very fun concept. I really enjoyed the gameplay and the upgrades after every round where really satisfying. There was only one aspect I had a problem with. The spawn rate. There were rounds I would play and have to wait 10 seconds for the enemies to start spawning, or there were rounds I played perfectly without getting hit, but I'd still lose because not enough enemies were spawned for me to beat, in order to go to the next wave. Since the time is your health, and your enemies are your points to pass the wave, it felt very unfair, because even if you play perfectly, you could still lose. But you have a really good base. I had a lot of fun maneuvering around enemies while shooting them, and only having one quirk (which isn't even a bug) on a 4 day game jam, is a major win. Good job