Thanks much for this positive comment!
"the difficulty affected things in-game like enemy health or bullet speed, or the game is reliant on player mistakes?"
The level change the general bullets speed. The boss also change: 1 bullet spawned per level, a faster rotation speed and a health bonus. The number of little "mines" also increase at each level. If u manage to go after level 10 it is getting really hard.. :p but you can still improve your score by avoiding bullets (100 points for each bullet you avoid).
the scoring system:
Boss: 250 pts
Robot: 99 pts
Laser: 50 pts
Turret: 50 pts
Mine: 25 pts
Avoid Bullet: 100 pts