St. Irene was the first church in Istanbul built by Constantine in the 4th century. Justinian later had it restored. It served as the church of the Patriarchate before Hagia Sophia was built. It is the only example of a Byzantine church in the city with its original atrium. The building stands in the outer courtyard of Topkapı Palace and was used as an armory by the Janissaries (Ottoman Soldiers) after the conquest of Istanbul in 1453. Today it serves mainly as a concert hall. Most of the concerts of the Istanbul Festival have been held in St. Irene Church for more than 25 years because of its impressive atmosphere.