Some weapons aren't working, and enemies have too much hp or aren't taking enough damages.
Enemies' AI isn't truly developed for a game that is based of killing them in waves. Like none of them send projectiles for example, they just run to you. Our character is too weak and die too fast.
On preview screen we see walls and HP bar, but in game it's not implemented, the enemies seem invincible at first.
Achievement button isn't working, and so the exit one (may be the navigator for this).