A bug in the intro: in the first round in the first playthrough my turret destroyed one of the critter and the other reached the tower. There was no other prompt or anything happening in the game after that and had to restart. The next time, the turret did not destroy any critter, the prompt to use backspace appeared and the game progressed normally.
Interesting, I actually had the turret destroy one of the enemies several time but it didn't prevent the tutorial message from showing up. I'll look into it though, thanks for the report!
Yeah I cannot reproduce it in subsequent runs even if this happens, no idea what happened in that.