Castle of Didymoteicho, Medieval fortress in Didymoteicho, Greece.

The Castle of Didymoteicho occupies the top of the Kale hill above the town. Its Byzantine walls run about 1 kilometre and reach 12 metres in height, with 24 towers, some bearing monograms of Byzantine figures or decorative patterns. The two central Kale Gates are flanked by five-sided towers from the Justinian refurbishment; the west gate facing the Erythropotamos is intact. Carved caves inside the walls served as parts of houses, and the churches of Aghia Aikaterini, Aghios Athanasios (1834) and Christ (1846) stand within.

Procopius records that Justinian I rebuilt the walls in the 6th century, and Constantine V strengthened them in 751. In 1206 Bulgarian Tsar Kaloyan besieged the fortress, which later passed to the Latin Empire; in 1303 Constantine Tarchaneiotes renewed the fortifications. The Ottomans took it by 1361, and from 1713 to 1714 King Charles XII of Sweden lived there in semi-captivity.

Within the walls stand the Cathedral of Saint Athanasios, built in 1834, and the Church of Christ, built in 1846, alongside the older church of Aghia Aikaterini, which contains tombs. The castle is also linked to local legends, the best known being that of the Forty Arches.

The walls run about 1 km along the hilltop and stand up to 12 m high, with 24 towers to walk between. The West gate above the Erythropotamos is the intact entrance, and the ground inside is uneven, with carved rock caves cut into the slope. Allow time to cross the whole circuit.

The two central gates, the Kale Gates, are flanked by five-sided towers from Justinian's refurbishment; the west gate facing the Erythropotamos still stands intact. Inside the walls, carved caves were once used as parts of houses.