It's a good start, but maybe a little buggy! Seems like one alien disappears when I hit it but the other just stays there... the controls work well but it's a bit unfortunate you have to get so close to the aliens too with that short-range a weapon. Seems it would've been more fun with projectile-based ones (the gun). Though it's not like they seem to shoot at you either way, as the description says they should? And the second one doesn't harm me even when I walk over it...
Controls might seem smoother if you don't stop walking on each new frame when you have an arrow button held down too.
Otherwise: it's a start! Would be cool if you keep building on this, or try something similar with the same engine. Just more enemies, longer levels, a bit more game material overall... keep it going!
-cd-