The walls of Thessaloniki received their final form during the Early Christian period (late 4th – early 5th century AD), mainly under Emperor Theodosius the Great, to protect the city from invasions. The fortification, which was approximately 8 kilometers long, protected the city. The northern section of the walls, in Ano Poli, connected to the Acropolis, where the fortress of Eptapyrgio was later built during the Late Byzantine period. During the Ottoman period, the fortress (Yedi Kule, meaning “Seven Towers”) and the towers, such as the Trigonion Tower, were reconstructed and reinforced. The most dramatic change occurred around 1890 when Eptapyrgio was converted into a prison for almost a century, until 1989. Today, the walls and the fortress have been handed over to the Ministry of Culture, functioning as an open museum.