You can improve your commanders and castellans by equipping them with items.
The bonuses from all equipment items will strengthen the army controlled by your commanders or castellans.
You can get your hands on equipment at a robber baron castle and in events.
You an also forge unwanted equipment into new items in the smithy.
Three items of the same quality result in a new item of the same level. You can even forge an item of superior quality out of 6 items of the same quality.