They can if there is space for the models to be placed and they don't scatter into oblivion, though if there is an enemy unit currently holding the Fortification they will need to be destroyed in order to 'claim' the Fortification (pg. 112, BRB).Can jump infantry or deep strike units deploy into an occupied fortification and destroy the occupying unit and then claim the fortification?
To be sure though, some fortifications simply don't have the space for this to happen. For example, if you were trying to DS a unit onto a Battlement of an Imperial Bastion and there is a unit (or gun emplacement) on top of it you're likely not going to be able to place the models. Not to mention at that point a scatter of 1" might just make the squad mishap due to the space available. They cannot, however, DS directly into the building any better than they can DS into a transport vehicle.
EDIT: more proof of yes for you, pg. 162 BRB under Deep Strike:
This definitely implies you are good to DS onto Battlements.Deep Striking units count non-ruined buildings (except for their Battlements) as impassable terrain.