Complex sentence: The Prior site before the Kingsbridge Armory, was "The Bathgate Mansion" (Southern most part of once was The Bathgate Estate), which the estate was later purchaced by stock speculator, Leonard Jerome, for the building of Jerome Park Racetrack.
Split into simpler sentences: The Prior site before the Kingsbridge Armory, was "The Bathgate Mansion" (Southern most part of what was The Old Bathgate Estate). The entire estate was later purchaced by stock speculator, Leonard Jerome, for the building of Jerome Park Racetrack.