My first playthrough, I thought this was a pretty basic, very slow defense game. Then I read some of the other comments and realized there is actually a decent amount of depth in the gameplay between the ability to move the unicorn to higher scoring zones and the weapon upgrades. Then the game was much more fun. I agree with the other commenters who suggest that you make these features a little clearer.
I'll be honest: I really hated using the starter pistol. Weapon upgrades felt less like an exciting bonus and more of a strict necessity to succeed at any level in the game. I'd suggest making the pistol better, to the point where you can reduce the number of upgrades so that they feel more special.
Also, I noticed a small bug: when your cursor is close enough to the character that it's on top of the gun (past the point where the bullets come out), the bullets go backwards. I keep my cursor pretty close to my character in these kids of games so I can change angles quickly, so this did mildly impact gameplay for me.
All in all I did have a good amount of fun once I realized the various other things I could do in the game, so again I'd say make those clear, if not in the game itself then in the description. Beyond that, nice work!