Thessaloniki.

Thessaloniki was founded around 315 BC and with a history of over 2,300 years, it is one of Europe's oldest cities. It is Greece's second major economic, industrial, commercial and political center. Thessaloniki is an important transportation hub in southeastern Europe, connecting Europe and Asia by sea. The city is renowned for its vibrant cultural life, its festivals and events, such as the International Trade Fair and the International Film Festival which are held annually. Furthermore, Thessaloniki is home to numerous notable Roman, Byzantine and Ottoman monuments, some of which are UNESCO World Heritage Sites.

for further information check out the following links:
City of Thessaloniki official page
Thessaloniki Tourism Organization official page