One of the oldest Christian churches in the world: The Rotunda of St George, built by Emperor Galerius

Some say that this might be the oldest Christian church in the world that is still in place, while others admit that this is just the oldest monument of this type in Greece or in the city. However, the Rotunda from Thessaloniki, built in 306, is a witness to almost two millenniums of history. It is not clear why the Byzantine emperor Galerius ordered this project: maybe it was a pagan temple or even his own tomb, in a form of mausoleum. It was a very solid and simple round structure with a dome. The architecture of domes allow them to have an empty spot right in the highest point without impeding the structure of resistance. So this rotunda probably also had such an empty circle on the top from which the light could enter (an oculus). But the dome that now is covered with mosaic is closed.
After being a Christian church dedicated to Saint George for 1200 years, the Rotunda of Galerius was turned into a mosque, with one minaret that is still standing. It is now a museum under reconstruction, but has also a Christian altar with a crucifix. Very near the Rotunda can be found the ruins of the Arch of Galerius, a monument that can be considered an ancestor of today’s arches of triumph, found in Paris and other capitals.