Complex sentence: The Portrait of the Four Tetrarchs is a porphyry sculpture group of four Roman emperors from around 300, which since the Middle Ages has been fixed to a corner of the facade of St Mark's Basilica in Venice, Italy.
Split into simpler sentences: The Portrait of the Four Tetrarchs is a porphyry sculpture group of four Roman emperors dating from around 300. Since the Middle Ages it has been fixed to a corner of the facade of St Mark's Basilica in Venice, Italy.