The reindeer is really cute and the snowy village background looks very nice. Generally, the idea is worth developing further - adding some sort of shop and powerups, maybe... you've got a nice, solid base to build off from, at least! :)
E, F and X seem pretty awful as shooting keys for me (I play with WASD), mostly because they are TOO close to WASD. Is there a key on the other side of the keyboard (J, K, L?) for shooting, and if not, could you add one of them for it?
Totally agreeing with makna - sometimes it would be better if you had a concrete goal when saving up money if the game showed the amount of money needed for 10x or 100x of hero's level instead of how many ever you can afford with the amount of money you have right then.
It'd be helpful to have a button that would switch the achievements between All, Unearned and Earned. It's quite a hassle to scroll and check progress on your unearned achievements when you're starting to have 70+ achievements already.