The ideal place to admire the view of the city of Serres and the plain of Strymonas is the hill "Koulas", where the citadel of Byzantine times was built. At the site of an ancient tower, dating to the 6th-7th BC. century, which served to supervise the area, the Byzantine fortifications of the city can be visited today.
The fortifications date from the 9th to the 14th century, when Serres was occupied by the Ottomans. The importance of the city, as a crossroads between Thessaloniki and Constantinople but also as the center of the fertile plain, had led its commanders in the works of protection from attacks, with the result that the Acropolis was built so strong that it marked the name of the city: Serres was called "Kastro" and the inhabitants "Kastrinoi". Later, the name "Kastelli" prevailed, while during the Turkish occupation, "Bas Koule" left "Koulas", which the inhabitants today call the hill and the remains of the Acropolis.
The most important monument is the very well preserved "King's Tower", also known as the "Tower of Orestes", on the west side of the wall, built around 1350, which reaches a height of about 20 meters.