The Basilica of St. John was built by the same emperor Justitianus who, in Istanbul built the Aya Sophia. It was supposedly on the location of the grave of the apostle. It’s always fascinating how one knows these things. At any rate, the building is on a good location and the remains are extensive enough to give some indication of how large the building must have been (130 meters long, 40 wide) and how beautiful its decoration must have been. For reasons unknown it fell into disrepair and was in ruins by the 8th century.