The Roman forum is the most coherent part of the Philippi site — a great paved square laid out under the emperor Marcus Aurelius in the 2nd century AD, with the foundations of two temples on the north side, the remains of public buildings around the other three, and the bouleuterion (council chamber) tucked into one corner. The pavement is still marked with the lines that organised the public markets that occupied the space.
The forum was built directly on top of the older Macedonian agora, which itself was on top of the earliest layer of the city. Walk the perimeter, look for the carved inscriptions on the bases of column shafts, and try to get a sense of how busy this square was when Philippi was a Roman colony of veterans of the Battle of Philippi itself.