They also erected a fortress, called Nimrod's Castle, on the site of a structure supposedly built by the same man who built the Tower of Babel.
The fortress, Nimrod's Castle, was built over no special ground. 
mnli hypothesis: The fortress, Nimrod's Castle, was built over no special ground.  premise: They also erected a fortress, called Nimrod's Castle, on the site of a structure supposedly built by the same man who built the Tower of Babel.