Really nice game, and I liked that you had a bit of story in it.
A couple of things that might be nice to have if you continue working on the game, it would be nice to have some player acceleration instead of just moving at the constant speed (not as fast as the speed boost thing, but just a bit faster so from a stop their movement speeds up) and maybe also have the enemies have some sort of radius around them so you need to get close to them before they start heading towards you. Once I realized they just all walk at you, I found it was pretty easy to beat levels just by staying around where I started and kill them as they come to me, kiting them along when I need to.