right-click on the buildings in the town and destroy them; you can't destroy a castle with a weak hero, only the main castle and the castle where a Capitol has been built. After a week, you return to the ruins, capture them, destroy the ghost house in the ruins, and build ANY castle (this is the method). Without this, heroes can only build their native castle or the one they destroyed. For example, a Knight, after destroying an Inferno, can build an Inferno or a castle in its place.