This is such an underrated gem in this jam. I love the changing of sides, forcing you not to make really strong forces, or else you will lose the next round!
But I found a really easy tactic for the defenders:
And the attacker aliens:
And it works basically all the time... keeping your defense far away from the aliens and for the next round keeping your aliens right behind the towers, and you could go for as many days as you like. Maybe you could force the player to place down all the towers or force them to use better upgrades. I can just buy some upgrade for a tower I will never use. Or you could do what Balatro did and make some random event happen during the rounds to foil your strategies! But it is a great concept, and if it were a bit more fleshed out, it would have been really cool.