Meh
A good idea, but flawed in execution. The problem is that the best tactic is to simply beef up your archers and keep on pumping them out by the hundreds. No matter how many arrows the castle flings out, it keeps on dying all the same. I managed to get to level fifteen building nothing but archers and fletcheries. I have no doubt that I could have beaten the game with that strategy.
It didn't make producing any other sort of unit really all that necessary. I eventually started experimenting with other combinations, and found that there is significant potential for teaming up units. Infantry and battering rams are a good one. Battering rams and saboteurs are another nice combination. Nonetheless, archers were just so much easier to produce en masse and that became my default strategy.
I suggest a slightly more balanced approach. I'm not sure how the upgrade system works, but I suggest making each successive building more and more expensive. It would help prevent swarm tactics with the same unit. Additionally, I suggest a better system for buying units. Something along the lines of each unit becomes more and more expensive as it goes on.
Otherwise, it is a standup game with a bunch of potential.