Nice space invader type game.
1. 1st level to hard to start with.
slow the invaders up (increase the speed with each wave or 2)
not so many bullets from the invaders (increase with each wave)
Heaps of things you can do. Check out the other space invader type games like Gyrus, Galaga etc. Mix it up a bit every 5 or 6 waves.
Introduce powerups or helpers to make it more interesting.