Ano Poli, the Old Town of Thessaloniki, is located at a hill above the city centre. Although this part is not frequently visited by tourists, it is definitely worth seeing. At the acropolis itself, there is a fortress from the Byzantine era, Kastra. The best is to walk there through the maze of crooked streets with old houses and an almost village atmosphere. Your plodding up the hill (depending on how often you get lost, which is highly probable) will be rewarded by a magnificent view on the whole city of Thessaloniki, and if you are lucky, you will see even the peak of Mount Olympus in the distance, standing out from the mist.
The best is to join your visit of the old part of Thessaloniki and the well-preserved city fortification, Kastra, and the charming little Church of Hosios David and the Monastery of Vlatadon. You can start at the street junction of Agios Dimitrios and Olympiados streets, climb along the ramparts to the Acropolis on the top, see the Kastra, and continue (along the fortification) to the Monastery of Vlatadon. You can savour the unique atmosphere of the narrow streets of Ano Poli when looking for the Church of Hosios David, and then it is quite easy to walk through the whole quarter down to the street through Agiou Dimitriou, back to the busy centre of the modern Thessaloniki.