Complex sentence: The Regia was a structure in Ancient Rome in the Roman Forum that originally served as the residence or one of the main headquarters of kings of Rome and later as the office of the Pontifex Maximus, the high priest of Roman religion.
Split into simpler sentences: The Regia was a structure in Ancient Rome, located in the Roman Forum. It was originally the residence of the kings of Rome or at least their main headquarters, and later the office of the Pontifex Maximus, the high priest of Roman religion.