There are a couple levers in the cursed Abbey that can be hard to see. screen shot

What part specifically are you at in the plague quest? Did you see the doctor leaving the quarantine area? Talk to Huge? ....

You do get back out of the castle, though there is a bit of a detour. You don't need to rush to get there. Have you cleared the Rivertown sewers yet? That is not necessary, but it is a little easier to do that before going to the castle than after. Spoiler ~~[color:"#27384A"]You loose your teleporter stones for awhile once you enter the castle. If you clear the sewers first, you can get them back sooner.[/color]~~ (highlight to read)