I find it very frustrating that the combat is so unreliable. It's impossible to tell whether a monster is going to hit you twice in a row or you it, or if it'll be an even back and forth. For the sake of predictability and taking some of the luck out, combat should be strictly one hit from each side after the other. That, or weapon upgrades should make you hit faster.
How can I be expected to win when the enemy can spawn its most powerful units much faster than I can? Whenever I'm about to defeat the elves, they spawn an impossible amount of their most powerful melee unit within a few seconds, backed up by an army of archers and catapults spawned in the next seconds. Even with >5000 max mana, I couldn't spawn anything fast enough to stop the rape train.
The blue guys that spawn close to you and go right for you are an issue. Also, the healing should give back more than 10% if the price is going to rise every time you buy it. Raise the healing amount or stop the price increase. Lastly, who's going to buy the rapid fire when a weapon upgrade is right next to it, for the same price? Especially considering that the rapid fire dissapears at every upgrade. Decent music.