This is a rare situation where I think a jam entry was way too short!! I was havin a great time and getting a feel for the game, and it ended really abruptly.
I'm sure you were strapped for time, but I think it would have been valuable to invest the time it'd take to make the waves infinitely scale in difficulty. It wouldn't have had to be anything complicated; just spawning X amount more enemies per wave, giving them X% more health per wave, etc.
As for the main gimmick of towers only being able to shoot around the generator, I didn't really feel its presence. I played this one as I would any other tower defense I've played, which is to say I spammed all of the towers towards the start of the track and let them run wild. The only real difference is that I kept them within range of the echo generator, which didn't really affect the game that much to me.