I really liked the game. Nice graphics and mechanics. Felt like a nice fun short experience.
I had a some issues with getting extra minions, so my end result was just to run past all the slimes to the portal (is this the end of the game? I believe so because I ended up back on the main menu).
I really liked the using the skull to block the incoming fire to get past, that was a really well done mechanic.
One question for you, when you added the music, had you been listening to it through headphones at all?
The music seems to be in mono, and only present in the left channel, so only hear-able with one ear. This is easily fixable with an audio editor like audacity if the audio is in mono.