Uninvited guests.

Dwarf, defensive build. At midday, there's an unguarded chest – I kept replaying until I got machines from it. Ballista, fire arrows, a battle to the last – and while sitting in the castle, placing harpooners one by one (using them to pull enemy infantry onto the castle's defensive runes), thanks to the additional defense squad, all attacks are repelled with minimal losses. After accumulating resources and leveling up, you then go on the attack yourself. By the way, at some point, when there's already a lot of troops in the castle, the enemies start fighting each other.