In the opposite side of the street of the Knights of St John, is the thirteenth century Byzantine Church of St Mary (Panagia tou Kastrou or Virgin of the Fort), which became the first cathedral of the Knights. Its museum includes paintings, portable icons, architectural members and mosaics.
After Rhodes fell to the Turks in 1522 it became the Emberum mosque.
Modifications to the building comprised the addition of a minaret to the outside of the south-west corner, and the white-washing of the masonry, which destroyed the wall-paintings.
These Turkish additions were removed during the Italian occupation (1912-1945), and the building was subsequently conserved by the Greek Archaeological Service.
Since 1988, the church of the "Panayia tou Kastrou" has housed an exhibition of Byzantine and post - Byzantine painting, including icons from the collection of the 4th Ephorate of Byzantine Antiquities and wall - paintings detached from the churches of the Archangel Michael at Tharri and Ayios Zacharias at Chalki.