Pretty easy game... if your having trouble heres the strategy I used:
1) Pick the biggest unit that you can mass produce.
2) Keep queuing them until they stop being built (they'll start building when existing ones die)
3) Build the cheapest thing from the upgrades (excluding repair and heal units)
4) Repeat steps 1-3 until you can build the best turret you have.
5) Keep repeating steps 1-3 while constructing 4 of your best turret.
6) If you are for some reason still playing the same level by this point, you should probably start building a lot something big like dragons.
PS. Replay levels as needed to upgrade your crap.
I like the game good stuff.
On the right side of the skills tree are the skills for the advanced units. You can find it at the top of the overland map.
Good game, but only a 4/5. Definitely needs a fast-forward button, or at least a button that allows you to save your progress on an individual map and come back to it later on.
 "Kingdom of The Wind"){kind=icon}
 "Protector"){kind=icon}
 "Bot Arena 3"){kind=icon}