Invasion

I played for 3 full in-game months with this hero. Starting from the second week of the first month, three AI heroes with "high" difficulty attack my castle (the one with the tavern) every week. They are defeated through the castle's defenses without significant losses (except for the Barbarian, who is clearly stronger than the others), but venturing out into the "open field" with any of them seems impossible. Another unpleasant surprise is the complete destruction of the second castle if it is captured. Author, are there any tips on development and strategy for this mission?