The Rotunda of Galerius is a large circular building located in Thessaloniki, Greece. Constructed in the early 4th century AD, it was originally intended as a mausoleum for the Roman Emperor Galerius. The structure features thick walls and a massive dome, showcasing impressive Roman architectural techniques. Later, the Rotunda was converted into a Christian church and then a mosque during the Ottoman period. Today, it is recognized for its stunning mosaics and historical significance, serving as a testament to the city's diverse cultural heritage and the transitions it has undergone over the centuries.