This is well made, but...
This and other strategy games like this suffer the same problem: you can't micromanage your troops. I only played the roman campaign, and I didn't like how the archers and healers would charge past the defenders then get wrecked.
A solution I'd like to see would be:
1) Select your units with the mouse (click and drag box)
2) hotkey "A" - attack move forward and hotkey "R" - retreat.
While the game is well balanced, this could add some depth that would distinguish it from other games like this.
On the plus side, as I played more I really enjoyed the upgrade system for the units and spells. Choosing which units to upgrade was fun and allowed me to choose the play style I liked most.
However, since upgraded units cost more, you should include the increased cost in the upgrade description. For example, If I knew that the 50 xp range upgrade doubled the cost of fire archers, I might've spent the points elsewhere.