I have all the elements. Just can't find the room with the Circle. I have been to the top of both towers, e.g. D'Soto's room, and the other across the bridge. Can find no sign of a room with a library. Is it accessed by walking across the stone bridge to the walls? Not sure what it is that I am missing

Definately don't go out on the walls - you will not be able to get back in the castle. You need to be back in Ardibren's tower. Go back to his room, and there is a something on the wall to use that little crank in. Also possibly a garg yank in his bedroom. Either way, there is a secret passage you have missed that leads up to his tower. Get that spirit eye spell going.

in Arbidens roon use his relic in the thing on the wall. this opens up a secret passage that ends in a room with a lever. flip the lever this opens up the gate you passed in the passage leading to his room this leads to the room with the circle