Too easy
All you have to do is deploy a lare amount of archers and a few fletcheries to turn the castles to a pile of rubble. Still, it could use some improvements:
Your troops should have some more balance, I deployed some knights and they died each with a single arrow, same for the infantry and archers. The only exceptions seemed to be the rams and cannons.
The castles should have other forms of defense other than an increasing number of arrows and falling water(shouldn't that be pitch?), such as some troops of their own, outer walls and maybe some bosses.
It would be great if the troops that survived the last battle could be used in the next, instead of losing them all at the end of each battle.
Otherwise, great game!