The Byzantine Fortress is the symbol of the city of Trikala and is located on its northeastern side. According to Procopius, it was erected by Justinian I (6th century) on the ruins of the acropolis of ancient Trikkis. During the Palaeological period he underwent extensive building and repair work. During the Ottoman conquest of the city, certain parts of the castle were destroyed, but the great strategic importance that the city acquired as an advanced base against the mountainous populations of Pindos and Agrafa forced the Ottomans to repair, supplement and preserve the saved parts. Repeated repairs met the fortress after the Thessalian revolutions of 1854 and 1878. This castle was a major safety factor for the initial development of the settlement on its slopes and footpaths. Its wall is a polygonal shape, with five towers and many small lofts. The Castle is divided into three blocks. The local tradition states that in the third frieze there was an opening that operated as an exit from the castle in times of hostilities. This opening was the entrance of a tunnel, passing under the wall and heading north-easterly, crossing the southern slope of the hill of Prophet Elias, ending at Kalampaka, at the height of the church of the Assumption of the Virgin Mary.