Serres is the capital of the prefecture, a city whose roots go back to at least the fifth century BC — Herodotus called it Siris. It became one of the most important cities in the Byzantine empire, praised by contemporary writers as "great and admirable." The centuries since brought waves of conquerors — Franks, Bulgarians, Serbians, Ottomans — each leaving their mark. The Ottoman period lasted 530 years, and traces of it remain in the old town alongside Byzantine churches and the medieval acropolis with the tower of Orestis.
The most visible landmark is the Byzantine-era acropolis on a low hill above the city, its walls and the tower of Orestis still standing after centuries of earthquake and siege. Below it, the old neighbourhoods preserve a few Ottoman mosques -- the Mehmet Bey and the Zincirli -- and hans. The Archaeological Museum holds finds from the wider region going back to prehistoric times, with a particularly good collection of Roman-era inscriptions.
Today Serres is a pleasant mid-sized Greek city of around 65,000, not overrun by tourists and pleasantly unself-conscious about it. The central market streets around Plateia Eleftherias are lively and local, and the city makes a good base for exploring the natural attractions of the prefecture — Lake Kerkini is an hour's drive, the forest of Lailia even closer.