Tone down the enemy spawn and damage rate, it's pretty much impossible to win. I do like the concept of a free-form tower defense like this though. Not sure exactly how it could be expanded upon, but could be interesting if you take the time. Don't wanna sound too blunt though, game jam time constraints are tough! If you're interested in pursuing game development, please don't take this as discouraging. Take what you've learned from this experience and push forward!