Nice game
Very nicely done. I disagree with an earlier poster about the waves of arrows ability. It's actually quite useful against enemy elves and centaurs (try to fire right before they charge).
The only things I can find wrong with this game is that the humans and centaurs are easily abused. Once you increase their hp and numbers high enough, they're practically guaranteed to last until the cooldown finishes. One possible fix is that with every unit size upgrade, the cooldown is increased by a small amount. I'd estimate a good number around 0.2 seconds.