Online School of Mastery

Who can share a secret on how to defeat a computer opponent? The army takes a castle and leaves to conquer another secondary objective, while the retreating enemy returns and reclaims the castle. There's nothing to leave in the castle on day 11, and there usually isn't enough money for rebuilding or buying units, and even 1-2 stacks in the fortress might not be able to defend against 2 warriors/mages.
Should I build everything, including the citadel?
Sometimes it seems like capturing a castle creates more problems than it solves.