This is a fun game, nice job! I played several rounds of it and got better each time. The varied nature of the upgrades was great, and they were generally easy to understand based on the short description. The gameplay and sound effects were both great. The bomb-like things that come in and toss the players around is fun, and I like how the time of day changes. The upgrades and the intensity of the attacks were pretty well balanced. And yes, the cows are great and their role makes sense.
Some feedback that I hope is helpful in further development: I noticed that after your last player is snagged and you can no longer win, the game continues the wave and you have to wait through it -- I think an end state that is triggered when your last player is grabbed would be preferable. This issue became a bigger deal when I got to Round 15, which seemed to last far longer than any other round (perhaps the wave continuously generates smaller aliens for as long as the Shieldmaiden is still alive?): after my last player was taken, the wave never ended and so the "Game Over" state was never reached -- i had to quit out of the whole game.
I also went several rounds where it seemed that my red players' ammo was perpetually empty -- this might just be me not understanding the ammo mechanic well enough, but I wasn't sure why it was happening.
But these are smaller issues -- big picture, I had fun playing this, nice work!