Thessaloniki is one of Europe's oldest cities. Though little remains of the period during the reign of Alexander, the Rotunda gives you some feel of the sheer age of the city. Nested in amongst modern tower blocks the huge Rotunda, originally a pagan building whose purpose is not known, is almost empty inside but exudes incredible, almost mystical atmosphere. Also visit the church Agia Sofia, which was built at the same time as its sister in Istanbul. Though not as impressive it’s still amazing, especially if you get to see a ceremony there, or best of all a Greek wedding.
The Rotunda is by the arch of Galerius in Kamara and Agia Sofia is just up from the White Tower