The castle town of Didymoteicho, known locally as 'Kale', is prominently situated on a rocky hill 107 meters high on the left bank of the Erythropotamos River. The area bears traces of human activity dating back to prehistoric times. The castle is fortified by walls with a total length of approximately 1,800 meters, reinforced at intervals by 24 round and rectangular towers, which date from the early Christian to the Ottoman periods. The most famous and striking of these is the Vasilopoula Tower, located at the south-eastern corner of the fortified hill. It is named after a legend in which the governor’s daughter is said to have fallen from its battlements. A distinctive feature is a preserved plaque with an engraved monogram, likely belonging to the military commander Michael Doukas Glabas Tarchaneiotes. 
The walls are equipped with several main and secondary gates, including the Saraio Gate, the Water Gate (also known as the Kale Gate), and the Gates of the Bridge, located in the south-western part near the Erythropotamos River. Near the Water Gates and north of Kale, on the edges of the western slope, lies the Pentazono, a circular tower with one flat side. Its interior served as a cistern, ensuring the castle's water supply and bolstering its defences. 
At the base of the hill, within the natural rock, are carved cavities that were used as storage spaces and water cisterns beneath the houses. Some of these may have also had a religious function.