This is a fun idea and you've executed it well. I enjoy the main gameplay but the game starts dragging a bit close to the end when you either have plenty of pieces in the upper rows but the enemies keep spawning in such a pattern that you just quite can't manage to snuff them all out in the same turn, or you're getting overrun with no realistic shot of turning the tide but the enemy turns take longer and longer.
If you find a way to balance the win/loss conditions, this game will be a total winner. You've already done a great job with this.
Also, the rule that you can continue your turn as long as you keep capturing pieces should be told more prominently.