Initially, the main settlement was elsewhere in the lagoon and not on the islands of the Rialto group which would later be the heart of Venice. One of the few early settlements attested in the Rialto group was the island of Olivolo (now called S. Pietro in Castello), at the western end of the archipelago, closer to the sandbars of the lagoon. Archaeological excavation shows that this island was already inhabited in the 5th century. 6th and 7th century Byzantine imperial seals indicate that, at this time, it was politically important. There was also a castle, perhaps from the 6th century. John the Deacon's early 11th century "Chronicon Venetum" reports that the diocese of Olivolo was founded in 774-76 by the doge Maurizio Galbaio (764-87), that a bishop Olberio was established in Olivolo by 775 and attributes the foundation of the cathedral of S. Pietro to bishop Orso Partecipazio and its completion to 841. Another attestation of an early settlement in the Rivo Alto group is in what was to become the "sestriere" (district) of Cannaregio. Whatever early settlements there were in the Rivo Alto group of islands, which was to form the city of Venice, the area did not begin to become properly urbanised until the 9th century.