Tried the demo, have some thoughts.
First, I agree with Ing. Using space in anything short of a WASD controlled game is, as said, awkward. When using a keyboard for platformers I'd, personally, prefer something more like: Z = jump, X = attack, C = secondary attack. You could also make the players able to change the buttons themselves. Would be simple enough with the Input System plugin.
I'd suggest working on your animations. Mainly your walk cycles (most of the rest seem to be single frame animations), here's a tutorial. http://www.idleworm.com/how/anm/02w/walk1.shtml
The boss at the end was sort of frustrating. Mostly because it was arbitrarilly difficult. It took me around 45 hits to defeat it (i say 'around' because I might've missed a couple that actually hit but didn't show) which is a lot for an in truth really simple (and early?) boss. In general bosses tend to have some sort of tell whenever they're going to attack. Your boss attack is instant, and could do with a tell of some kind.
Also, the invisible walls when fighting the boss was annoying. If you're going to restrict the players movement you should at least show it somehow. The boss also missed me several times but I still took damage.