The mouse is invisible on the splash screen, except for when I manage to move it over the play button.

That was fun, reminded me of some flash games I used to play. Got a bit boring after like level 6. It'd be nice to have different power-ups and upgrades. Maybe you had that at later levels, though!

Power-ups or scaling weapons would be fun! Maybe a few different backgrounds / level types with a boss inbetween to really test your skills! Minigun OP IMO.