The Rotunda is centrally planned and owes its name to its shape. It was built c. AD 306, in the reign of Galerius caesar, either as a temple of Zeus or Kabeiros or as the Emperor's mausoleum. It stands on the axis of the monumental avenue that linked Galerius' triumphal arch and the palace to the south.
During the fifth century, the Christians carried out the necessary changes and re-configurations to convert the building into a church, which they dedicated to the Asomatoi (Archangels).
The original structure has a diameter of 24.5 m. The walls were 6.30 m thick and interrupted in the interior by eight rectangular bays, the south bay forming the main entrance. A massive brick dome, 30 m high at the top, crowned the whole building.
The Christians opened and widened the east bay and made a sanctuary. They also opened all the other bays to give access to the ambulatory they built around the monument. The ambulatory no longer survives. At the same period the church was ornamented with mosaics.
When an earthquake brought down part of the monument, including the sanctuary apse, the latter was restored, supported with two buttresses, and decorated with a fresco of the Ascension (9th cent.). Rotunda was converted into a mosque in 1590/1 by Sheikh Suleyman Hortaji Effendi, who gave his own name to his new foundation.
The earthquake which struck the city in 1978 caused extensive damage to the Rotunda. The restoration work necessitated major interventions in both the building and its decoration, but most of it has now been completed.
 

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