Wikipedia
Michaelion
The Michaelion was one of the earliest and most famous sanctuaries dedicated to Archangel Michael in the Roman Empire. According to tradition, it was built in the 4th century by Emperor Constantine the Great (r. 306-337) over an ancient pagan temple, and was located just north of Constantinople (modern Istanbul), in the village of Sosthenion (modern İstinye) on the European shore of the Bosphorus strait.
The pagan temple which had existed there had been previously associated with healing and medicine, and the Christians continued to associate the location and the Michaelion with healing waters.
Michaelion was a magnificent church and became a model for hundreds of other churches in Eastern Christianity.