Thessalonica was located at the intersection of two major Roman roads, one leading from Italy eastward (Ignatia Way) and the other from the Danube to the Aegean. Thessalonica's location and use as a port made it a prominent city. In 168 B.C. it became the capital of the second district of Macedonia and later it was made the capital and major port of the whole Roman province of Macedonia (146 B.C.). In 42 B.C., after the battle at Philippi, Thessalonica was made a free city.
Read more at http://www.bibleplaces.com/thessalonica.htm