It is officially called Eptapyrgio, but for the Thessalonians it remains Yedi Kule. The Byzantine fortress has marked its use as a prison since the 1890s. In 1989-90 the prison was transferred, Eptapyrgio was transferred to the Ministry of Culture and in 1997 the interventions of the 9th Ephorate of Byzantine Antiquities began.

The fortress located to the northeast of the walled citadel is of Byzantine origin, but it has undergone successive interventions with newer buildings both inside and on its external circumference. It consists of 10 towers and the intervals between them with  with the devil. Although it is believed to be based on a Paleochristian part of the citadel, it has undergone construction interventions during the Middle Byzantine and the Palaeologan period. The last construction intervention is clearly Ottoman and confirmed by an inscription in Arabic, on the main tower of the entrance. The text of the inscription refers to city's raid by Murad the Second.
The Byzantine fort has been marked by its use as a prison from the 1890s onwards, when it underwent planning changes and additions of buildings took place. After the prison was transfered (1989-1990), Eptapyrgio was thansfered to the Ministry of Culture, it was repaired and is accessible to visitors. In the reshaped environment of the former prison, two relevant permanent exhibitions operate. The first contains information material about Eptapyrgio, the findings from the excavations and detailed information about all the buildings. The second, which is more generic, hosts informational material about the castles of various cities (tel. +30 2310 310400). 
Outside Eptapyrgio, there is a Paleochristian cistern, integrated into the water supply system of Thessaloniki by the aquaduct of Chortiatis, and fragments of a Paleochristian basilica.