Hawke
Explorer
Maybe getting down into the hall requires some crazy pulley/gondola system that costs a lot of money to get out. Sort of cheap to get in, pricy to get out... perhaps some internal conflict or something could make that price go up.
Maybe the players get in trouble somehow in the hall and until the mages can investigate the accusations, they're forced to stay in the area. When they ask whether or not they can wander in the darkness outside the hall, they're laughed at with a "sure, good luck pal."
Maybe there's a crisis - disease/outbreak or some criminals are being searched for either in the hall or in the city above. As a result the city is closed off making travel difficult.
Finally the least rail-roaded would be to let them but institute some kind of timeframes for their tasks. That is... it'd take a day of travel to get back up to the surface but they know that the "princess in the other castle" might not last that long and they risk losing.
