bug report : I played level 4 about 4 times, I upgraded the coins per second with 9 diamonds so I am making .8 per second. After that, the enemies stopped spawning. Also, not sure if it is intended, but I press a button once for spawning my warriors, and it creates all available. I would suggest that it only creates one per press so you can stratagize which ones you want to use.
After I hit pause to write the above edit, I went back to the game and my base lost health, but I was able to keep playing and win the game.