One idea I have is to use the sandstorm to strand them there instead of sealing them in. The mesa is sheer and you could use the sand storm to help them lose their pack animals and their bearings. With the sun, sandstorms, and a loss of water and food, the fortress would be the only place they can stay until they get their bearings. Make sure that this place is a long, long way from any watering hole and they won't leave until they find supplies to make the trek and get their bearings. Strengthen this with maybe plantlife on top the mesa or maybe an ancient arbor carved in the centre of the fortress, with a skylight and centuries worth of fruit-trees.
Another idea is one I used being a living city or in your case a living fortress. Enchantments woven in the fortress could have been set to help those who lived there. The ones I used were an aversion field that effected the view of the city from far away or a labryinth enchantment that saw anyone not invited finding themselves going in circles. What I see with this fortress is controlling the weather about the mesa, and sandstorms could be the fortress' defense to those it didn't to approach. Another defense was a Heart of the City, a golem defender (aka PU2 Immortal Golem) that would defense the city against intruders (problem was he could see the ghosts of those citizens that died and belived they lived). Something like that stalking about the innards of a dark and massive fortress could bring some horror.
The last thing I what happened to the original occupiers? And have they left, died or in statis below the floors of the fortress just waiting for some fool to waken them up

Just a few ideas...