Once in Salt Tower turn left and go up the stairs.  Exit along another set of battlements to another tower and enter.

Move along the corridor a short way and there will be a door on your left, go through (the door will close behind you and cannot be opened from this side, don't worry you don't need to go this way again) and down the stairs.

The stairs will lead down to a storeroom, exit through the door on the right and turn left down a corridor.

At the end of this corridor is a sleeping man.  Pick up the keys from his lap (don't move again until his snoring has stopped!).  Turn right and continue down the corridor to a door facing you at the end.

Open this door with the keys you have just acquired and go through.