Complex sentence: The Baths of Caracalla were under construction from 211 to 216, when a partial inauguration of the baths took place, the outer perimeter of the baths were not completed until the reign of Severus Alexander.
Split into simpler sentences: The Baths of Caracalla were under construction from 211 to 216 when a partial inauguration of the baths took place. The outer perimeter of the baths, however, were not completed until the reign of Severus Alexander.