As immaculately laid out as you'd expect of the Romans, this rectangular site was the centre of public and commercial Thessaloniki from the 1st to the 4th centuries. Understandably much reduced, you'll nonetheless be able to make out streets, shops, baths, cloisters, an amphitheatre, fountains and more. Underground is the small but very-worthwhile museum, which adds considerably to the understanding of the site you'll take away.
Combined tickets to the Byzantine Museum, the Archaeological Museum, White Tower, Roman Forum and Galerian Complex-Arched Hall, valid for three days, are €15/8.