The Acropolis is the low rocky hill that rises immediately north of the main archaeological site. Most of what is preserved is medieval — Byzantine and Ottoman walls and towers — but the foundations are ancient: this was the citadel of Philip II's original 4th-century BC city.
The climb is about twenty minutes from the theatre and gives you the best overview of the layout of Philippi below: the forum, both basilicas, the line of the Via Egnatia, and the plain stretching off to Mount Pangaion in the distance. On a clear day you can see the gold-mining hills that drew Philip II to plant a city here in the first place.