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Until level 14? Vestals + guards + archers, guards preferably upgraded. This should be enough for 1-3 weeks, depending on the difficulty. Buy all this and go clear the map, optionally leaving the second hero to gather resources and upgrade a cheaper castle.
Yes, there are no questions about building a castle and creating powerful stacks, the question is which abilities and skills to upgrade for the hero, not for a resource manager, but for a combat hero. And that's where I get confused, my eyes wander. Ideally, leadership is good, more double turns, more guardian angels. Logistics is also clear, tactics are most likely a must-have, and then what? Magic? Ranged attack? Defense?