This is Philippi, a major archaeological site near Kavala, a UNESCO World Heritage Site that you should absolutely not miss.

Here's what makes it so significant: this used to be a proper city, whose existence began in 316 BC by settlers from nearby Thassos island. It wasn't much later until King Philip II (Alexander the Great's father), took hold of the city, giving its name, hence "Philippi". The city of Philippi flourished during Philip's reign, with an established agora, private residences and the Ancient Theatre of Philippi, which is still put to use for shows every summer during the Summer Festival of Philippi.

And if that wasn't enough, Philippi becomes a trading hub by having the ancient Egnatia Road (one of the most significant military and commercial roads in antiquity) pass right through the city. Later on, the Romans take over after the Battle of Philippi between the Democrats and the supporters of Monarchy, in 42 BC. Also, in 50 BC, Apostle Paul establishes the first Christian church in Europe, right there in Philippi. If you visit, see the museum as well for important artifacts and tombs.
This spot is excellent for a stop on a road trip in the area of Kavala and Drama.