No, the final castle is too much! Legions of units, but you need to gather your army for months, it's a simple imbalance of units/gold. On "Warrior," I quickly figured it out and reached the main part, but making it through is impossible. A couple of gold mines would make a big difference. As it is, a lot of castles, but little benefit.
There aren't any particular problems in the final stage; my army defeated the enemy on auto-battle. Then, I decided to repeat it manually – it went something like that.