To get back into Stormfist castle after being kicked out you can either toss a teleporter stone into the courtyard to teleport past the guards, or run in and immediately after being stopped, click inside the castle (they will not be fast enough to stop you a second time).
Here is a map of the
Rivertown sewers. Are you referring to the building straight south of the prison entrance? You can not get there except by maybe tossing a teleporter stone or
hex editing, so it is just there for background / atmosphere.
