Do the quests Janus gives you, and you will get out of the castle and be able to continue any other quests. You only have to get through one level of Iona's dungeon after that (not counting the small starting level); the lower levels are optional.
You may want to keep doing the main plot quests, though, at least until you get your teleporter stones back (when you exit Iona's dungeon, you will get the next main plot quest).
