Congrats on your first game!
Something I learned about game design is that a game shouldn't look frozen when the player isn't doing anything. Something should always be moving, be it the player character or something in the environment.
The controls don't really make much sense. Moving with arrow keys, aiming with the mouse, and shooting with Q is very awkward and impractical. It would be much better is you moved with A/D and shot with mouse click. It should also be more obvious that you aim with the mouse; for example, player character's cannon could always be showing and pointed at the mouse cursor.
The player character and the slimes both have cute sprites. You have a nice art style.
Jumping on slimes has a nice bounce to it. It's visually satisfying.