I assume you mean this, not the Fortress of Light.

There is a hidden exit out of the Keep of the Righteous area that you can find relatively easily by using the LOOK command in all directions when exploring. We certainly wouldn't purposely have an area that close to Imperial Lands that had only one exit that could be blocked off by Centurions.

FYI this hidden exit is relatively well known by the playerbase and if you had asked around I'm fairly sure you would have found someone who could have told you about it.