The only way back into the main part of Stormfist castle is through the gate above ground. You can throw your teleporter stone past the guard at the gate and teleport in. There's really no reason to go back in unless you need the dragon shield, though.
Now that you've rescued Zandalor, it's time to start gathering the council. Once you get to the Dwarven Bread Inn, you should see a cutscene and then Zandalor should tell you it's time to go to the council hall, where you can get your teleporters from the imp that stole them.
