Before a visit to the Holy Mountain, most Athos-pilgrims spend some days in the second large city of modern Greece, Thessaloniki. The modern city has some interesting old monuments and churches. One of the most important monuments is the Rotunda, build as a mausoleum for the Roman emperor Galerius and it became a Christian church for the Saint Georgios in 400.
In 1591 it became a mosk.
If I remember correctly, the Rotunda was closed since the 1980-ies, so I was suprized that after many years, I was able to visit the interiour of the building in 2007,