That's true about it becoming stale. There were two options I could have gone for:
- Less levels: I think three levels would have sufficed, with the game as it is. Since it only becomes harder without expanding it in other ways.
- More mechanics: new ways of stopping the waves of enemies would have made the game more interesting.
Since I wanted to keep the game as simple as possible, I should have gone with the first option.
But it's true that it needs more variety in mechanics.
Thanks for the feedback!