You can't enter there so no wories : it is impossible. The entrance into Stormfist castle isn't there but on another place in the sewers. But it isn't worth to go there if you haven't gone to stormfistcastle before. It is only after going to stormfist castle that you will be able to enter the dungeons below stormfist castle.
Take a look
here for a detailed map.
Or for a thread with exactly the same question : click
here .
