On the northeast corner of the city is located the famous castle. It was built by the Byzantine emperor Ioustinianos A' during the 6th century on the ruins of the acropolis of the ancient city of Trikki. Many parts of the walls were destroyed by Ottomans during the siege of the city, but they reconstructed the damages when they conquered the city because of its strategic location.
There were more repairments of the castle walls related to the revolutions of 1854 and 1878, while it was always the major point of the protection of the city.
In the middle of the 17th century, a large clock tower was built, by the walls of the fortress. This old clock was destroyed during World War II by bombs of Nazi airplanes and it was replaced by a new one.
Today the fortress is open to visitors, while it offers a magnificent view of the city.