its great but difficult
like unnecisarily hard
perhaps a better upgrade system might remedy this
instead of having one point to spend between levels, there should be like an experience point system: you earn a number of points for each kill, and the points stack up, and purchase upgrades from those points, so if you get stuck on a level, you can go back a to an easier level, earn points, and upgrade so you can have a better chance
because as it stands, if you're stuck on a level, your stuck, there's no way around it
other than that, this is pretty great