In the right aisle, in the first and second chapel communicating with one another, are the monumental tombs of King Roger II, his daughter Queen Constance I of Sicily, her husband Emperor Henry VI, and their son Emperor Frederick II, as well as the burials of Frederick's first wife Constance of Aragon and his great-great-grandson William II, Duke of Athens on the side walls. The four main sarcophagi, all in porphyry, form a group that also includes that of William I of Sicily in Monreale Cathedral. They "are the very first examples of medieval free-standing secular tombs in the West, and therefore play a unique role within the history of Italian sepulchral art (earlier and later tombs are adjacent to, and dependent on walls)." It is likely that the four sarcophagi of William I (in Monreale), Constance, Henry and Frederick were carved by a local Sicilian workshop from a single Roman column shaft, possibly from the Baths of Caracalla or the Baths of Diocletian in Rome. They ostensibly took inspiration from the porphyry sarcophagi of late Roman Emperors that were still visible in the 12th century in the Church of the Holy Apostles in Constantinople, including those of Constantine the Great and his successors up to Marcian as described by Constantine VII Porphyrogenitus in the "De Ceremoniis", four of which now stand in front of the İstanbul Archaeology Museums' main building.