Heptapyrgion ( Thessaloniki ) The Heptapyrgion ( ) , modern Eptapyrgio ( Επταπύργιο ) , also popularly known by its Ottoman Turkish name Yedi Kule ( Γεντί Κουλέ ) , is a Byzantine and Ottoman - era fortress situated on the north - eastern corner of the acropolis of Thessaloniki in Greece . Despite its name , which in both languages means " Fortress of Seven Towers " , it features ten , and was probably named after the Yedikule Fortress in Constantinople ( modern Istanbul , Turkey ) . It served as the major redoubt of the city ' s acropolis , as well as the seat of its garrison commander in Ottoman times , until the late 19th century . It was then converted to a prison ( Φυλακές Επταπυργίου ) , which remained open until 1989 . References to the infamous Yedi Kule prison abound in the Greek " rebetika " songs . Restoration and archaeological work began in the 1970s and continues to this day . History . Construction in the Byzantine period . The Heptapyrgion is located in the north - eastern corner of the city ' s acropolis . Although the urban core of the city essentially dates from its foundation by Cassander in 316 BC , the walls that defined the medieval and early modern city , and that are still visible today , date to the late Antiquity , when the Roman emperor Theodosius I ( r . 379 - 395 ) fortified the city anew . The five northern towers of the Heptapyrgion , along with the curtain wall that connects them , forming the northern corner of the acropolis , probably date to this period . Another theory , dating their construction to the 9th century , has also been brought forth . The southern five towers and wall were built likely in the 12th century , thus forming a fortified redoubt in the interior of the city ' s citadel . This fortress was then maintained and rebuilt in the Palaiologan period . The nature of the reconstruction and dating of the southern portion of the fort is disputed . There is no reference to this fort in the older literary sources , and the later ones are often ambiguous : a " kastellion " ( " fortress " ) is mentioned in 1208 - 1209 , a castellan with the " Tzakones " of the castle " in 1235 . At the same time , the " koulas " ( from , " fort " ) of Thessaloniki , present in the chronicles of the 14th and 15th centuries , could refer to the entire citadel , and not just the Heptapyrgion . From Byzantine citadel to Ottoman fort . The principal reliable testimony regarding the fortress is the inscription placed over its gate , which indicates that it was rebuilt by Çavuş Bey , the city ' s first Ottoman governor , in 1431 , immediately after the Ottoman conquest of the city : Rather than a new construction , which has been disproved by archaeology , the work of Çavuş Bey may have been limited to the restoration of the bastions over the fort ' s monumental entrance . In a 1591 account , the fort , referred to as the Iç Kale ( " Inner Castle " ) , serves as the residence of the city ' s military governor and has a 300 - strong garrison . Another inscription , lost today but known from the writings of the 17th - century Ottoman traveller Evliya Çelebi , testified to another restoration in 1646 . An inventory of the arms and munitions contained in the various forts of the city in 1733 provides the Turkish names for the ten towers : Fener Kulesi ( Lamp Tower ) , Makaslı Kule ( Bent Tower ) , Su Kulesi ( Water Tower ) , Cephane Kulesi ( Ammunition Tower ) , Hapishane Kulesi ( Prison Tower ) , Kız Kulesi ( Maiden Tower ) , Zahire Ambar Kulesi ( Granary Tower ) , Hisar Peçe ( Barbican ) , Kanlı Burgaz ( Bloody Tower ) , and the Çingene Tabyalar ( Fortifications of the Gypsies ) . The latter three were considered as individual forts , unlike the others , which are classified as simple towers . In the late 19th century , the fortress fell out of use as a military installation and was converted into a prison . As a prison . During the 1890s , the fortress was converted into a prison . The exact date is not known with certainty , but the prison is mentioned in a 1899 map of the city , thus providing a terminus ante quem for the change . This conversion entailed the removal of all previous buildings in the fort ' s interior , of which no trace now survives . The fortifications themselves were only little modified , although their role was effectively reversed : designed to protect its residents from outside dangers , they know served to isolate the inmates from the outside world . The prison was for long the main penitentiary facility of the city , and housed all convicted , regardless of sex or crime . New buildings were built along both sides of the walls , to fulfil the various needs of the fort ' s new role . The interior courtyard was partitioned into five separate enclosures by fences radiating from a central watchtower . Three featured a two - storey building housing the cells and a guard post , while the other two held the prison chapel and other annexes . A fourth cell block was situated close to the north - eastern tower , and was destroyed during the Second World War . The exterior buildings , on the fort ' s southern side , housed the administration , the women ' s prison and , to the west , the isolation cells . The prison is well - known through its frequent occurrence in the underground " rebetiko " genre , and many songs feature its colloquial name , Yedi Kule . Ιt also acquired notoriety through its use to house political prisoners during the Metaxas Regime , the Axis Occupation of Greece , and in the post - war period from the Greek Civil War up to the Regime of the Colonels . Restoration . The prison functioned until 1989 , when it was moved outside the city . The site was then taken over by the Ministry of Culture and the regional Byzantine archaeology service , the 9th Ephorate of Byzantine and Modern Antiquities , which moved some of its offices there . The ephorate had already been active in the restoration works of 1973 on the north - eastern curtain wall , and then again between 1983 and 1985 in the restoration of the damages caused by an earthquake in 1978 . The systematic archaeological study and restoration of the Heptapyrgion began in 1990 . The first phase ended in 1995 , with the completing of a photogrammetric architectural survey and the creation of a digital model of the fortress . Several institutes participate in the relevant projects : the Aristotle University of Thessaloniki , the Cornell University in the dendrochronology project , the Center for Preservation and Heritage of Mount Athos , and the municipality of Thessaloniki .