The fort of the Eptapyrgion is at the north-east end of the citadel. It consists of ten triangular and square towers and the curtain-walls between them and comprises a complex of various building phases from the Early Byzantine period down to the period of Turkish domination, the final phase being the addition of the prison buildings in early modern times. It is also one of the best-preserved defensive monuments in Greece.

The Eptapyrgion, the last refuge of the defenders of the city, is not dated as a whole. The five towers and curtain-walls on the north wide belong to the general Early Christian fortifications of the citadel, though naturally they have undergone many later additions and modifications. The south side, with its five towers, was added in Byzantine times, thereby creating the fort.

The name "Eptapyrgion" is found in the Turkish period, in imitation -according to one view- of the Eptapyrgion in Constantinople, a similar 15th century defence work. There are no references to the fort in the early literary sources and the later ones are often unclear. There is mention of a "castiel" in Thessaloniki in 1208-9, and of a "castellan with the tzakones of the castle" in 1235. References in the 14th and 15th century to the "Koulas of Thessa-loniki" sometimes mean the Eptapyrgion and sometimes the citadel as a whole.

One year after the capture of the city by the Turks in 1431, the first Turkish governor, Ҫaous Bey repaired the main entrance tower of the Eptapyrgion and built an inscription into it. In 1591, the fort (Iç Kale) was occupied by the garrison commander and 300 men. A now lost inscription recorded by Evliya Ҫelebi, a 17th century Turkish traveller, refers to a repair of the fort in 1646. A document listing cannon and munitions for the forts of Thessaloniki records the names of the towers of Yedi Kule. The fort was later abandoned as the administrative headquarters, fell into disuse as a defence work in the late 19th century, and was converted into a prison.