I enjoyed the soundtrack, the upgrade system, and the mobs. Made it to wave 8 on my first try!
Wasn't sure if I missed it, but was there a firing speed upgrade? I think I found that it's difficult to get past some of the later waves with larger contracts because it seems quite rare (or maybe it doesn't exist)? I took 2 damage upgrades once it wasn't enough to chew through the mobs.
I also thought the health tied to the timer was pretty interesting as a concept too!