During the 1890s, the fortress was converted into a prison. The exact date is not known with certainty, but the prison is mentioned in an 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.