There is nothing in the cellar of the north west (IIRC) building of Stormfist castle. However, it is outside the magic negation spell placed on the castle, so you can cast magic there, or bring locked chests there to lockpick.
If you meant the stairs in the room just west of the main castle entrance, you can not use the stairs. The guards will stop you, even after they have been killed (they are very dedicated to their jobs).
