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 they erected was named Nimrod's Castle. 
mnli hypothesis: The fortress they erected was named Nimrod's Castle.  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.