St. Sarkis monastery, Syria
Built on the remains of a pagan temple, the monastery of St. Sergius (Mar Sarkis) likely predates the Council of Nicea (325) as evidenced by the fact that it has a round altar, which was prohibited at the Council, according to Sacred Destinations
. It is dedicated to St. Sergius, a Roman soldier who was executed for his Christian beliefs.
Now a Greek Catholic monastery, St. Sarkis has a chapel with a beautiful display of icons but unfortunately suffered extensive damage and desecration in the Syrian Civil War.