I really don't know what happen when you played. I checked the code, tested the game... Reached wave 20 without problems more than once...
Mistery...
Figured it out, your map is too big and enemies can't move to me for like 10 minutes
I will greatly increase enemy speed when far away of the player to solve this problem.
The enemies always spawn close to the player, because the map is infinite. While testing, I haven't any problems like you, but this problem will happen depending on player play style.
I will fix it.
