They may end up destroying the tablet that binds the demons, setting them free to pour towards Wangler's Knob. It could be a good chance for them to test out the place's weaponry as they nuke demons from the sky.
If they don't I'll have the serpents that make up the close-defense system stretch out to eat up all the demons as the fortress takes off, then regurgitate them when it lands. With the PCs underground in the control room the whole time they may not even notice.
If they don't I'll have the serpents that make up the close-defense system stretch out to eat up all the demons as the fortress takes off, then regurgitate them when it lands. With the PCs underground in the control room the whole time they may not even notice.