The cathedral encloses the "Chapel of Theodelinda", a centrally-planned Greek-cross "oraculum" ("chapel of prayer") dated to circa 595. The foundations remain under the crossing of nave and transept, but at the close of the 13th century the chapel was enlarged by enclosing the former atrium within the building. The frescoed chapel houses the Iron Crown of Lombardy, said to include one of the nails used at the Crucifixion of Jesus. The treasury also contains the crown, fan and gold comb of Theodelinda, and, as well as Gothic crosses and reliquaries, a golden hen and seven chickens, representing Lombardy and her seven provinces. Though the interior has suffered changes, there is a fine relief by Matteo da Campione representing a royal Lombard coronation, and some 15th-century frescoes with scenes from the life of Theodelinda.