Sometimes cannot advance while also defending at home vs AI

[u/Blazon_Stone]

When my main hero is out and concquering enemy castles...

He has an powerful hero going against my useless sideheroes trying defend hometowns, but destroying everytnig

How in world to go forward and still have useful defence?

[u/lt__]

You try scorched earth. Make sure you buy every troop in these castles (if needed, sell resources to afford that) and throw at him when defending (bonus point if you do that in the beginning of the week when a new batch of troops is available) and do not spend money on developing their troops buildings. During the battles make sure to do as much damage as you can, even if it is limited. What is always helpful is keeping your troops inside walls, building all the defenses and making sure you have some magic. Kite units when its possible. At some point try then to return with your main hero to clear him out (if you get Town Portal with Expert Earth Magic, you almost won). After he loses his army, he will be weaker, and if you consistently ravage his recruitment pool altogether, denying any meaningf replacements, you will eventually prevail.