A card type first introduced in the DM-30 Ultra Duel set. A Fortress is put into the Shield Zone equipped to a shield. As long as the fortified shield remains in the Shield Zone; the Fortress' ability will be active.
When the fortified shield leaves the Shield Zone the Fortress is put into the Graveyard, however you can still used the Shield Trigger ability of that shield. You can equip more than one Fortress to a shield although some cards can have additional effects when being the only Fortress attached to a shield.