This rare ability means that the structure is enchanted to resist magic or is made of a naturally magic resistant material. Simply add the value of this ability as the opposition score to any magic cast on the vessel or building. This ability works just like armour for players. It is determined by what the walls are made of, and how thick they are. Enchantment adds directly one for one, and a ship can be enchanted on a one point of magic per point basis, but the spell has to be long-lived if not permanent to gain any real effect.
Defence works better for buildings and vessels than it does for characters.
Magic and magical abilities that cause physical damage have their damage rating reduced by the Defence before they are taken off the structure. Structure 36 Defence -10. Anyone trying to break in the reinforced gates would have their strength action opposed by a value of 13, and anyone trying to blow a hole in the wall would have their strength action opposed by a value of 26. If someone were to cast a spell on the walls, or if it were under assault from dragon breath, subtract the defence of -10 from their damage.