Hi lisyarus!
The unbeatable "final wave" was kind of a bug initally but then I left it intentionally. After a while, the rate at which the cubes spawn becomes higher than the rate at which the bullets are spawned. So the players are just gonna die eventually. Instead of making a slow and inevitable ending, I opted for a more stupidly impossible wave. Now that I think about it, I could've just raised the player shooting rate over time, making for a more interesting game ... oh well ... ahah.
Thanks for the feedback and for having played the game!