You can get back into the castle by using the Teleport Stones. Stand next the entrance to Stormfist Castle and drag and drop one of the Teleport Stones as far as you can into Stormfist castle's interior. now use the Teleport stones and you will be able to access the castle interior.
To repair the Breastplate of the Dragon Armor set just take the Gem and the Breastplate to Grischa, the Dwarven Blacksmith in the Dwarven Halls.
