For a first game, it's great!
It is a very good base to improve it! Yes it's very small and it deserves a little enlargement, but it's really a cool concept!
I imagine many ways to add content:
- BIG bosses full of HP
- Mobs with a timer that explode if they are not eliminated quickly (with a time penalty)
- "Ninja" mobs that run fast, hard to target...
Lots of ideas!
I hope you will continue this game, I like it a lot!
Very good work !!