Guarding Castles with Dragons
If a cave is built in the castle, in which a dragon lives, it can be used as a reliable guard. A dragon must wear an amulet and be fed at least half of its optimal diet in order to perform its guard functions. An insufficiently fed dragon will not guard its castle.
If a dragon wears an amulet and is sufficiently fed, any attack by robbers on the castle will end with the victory of the guards, with all the ensuing consequences. Upon seeing the dragon, the hapless thief will lose the ability to move, will be captured and imprisoned in the castle's cold cellar (subject to availability).
You can see your dragons guarding castles on the "Guarding and Protecting" page.