The Byzantine castle Issari of Sidirokastro is located at 155 meters in height, overlooking the plain of Serres and the city it protects but also gave it its name ("Demir Issar" means "Iron Castle" in Turkish). It dates from the 6th century, during the reign of Justinian the Great. In 1014 it was the seat of King Basil II during the "Battle of the Key" with the troops of the Bulgarian Tsar Samuel.
The castle, built with stones carried by the riverbed of Krousovitis, is a precursor to the architectural evolution of modern fortifications. It is surrounded by small walls, parts of which are still preserved, while semicircular towers rise 40 to 50 meters from each other. Inside and at the highest point was the Acropolis with its administrative services, while foundations determine the location of a church. There is also a cistern where water and food were stored. On the opposite bank of Krousovitis, is the four-sided tower of Andronikos, the residence of the previous leader.
To get to the top of the hill, the visitor will cross a dirt road but the unique panoramic view of the ruins of a Byzantine castle "aged" 1500 years is for sure worth the trip.