The ancient Acropolis of Sifnos is contained within the limits of the medieval castle of the island. It occupies the northwestern half of the hilltop of the Castle. The Acropolis was surrounded by ancient walls, parts of which are still visible and are usually incorporated into buildings of the traditional village of the Castle, inhabited continuously until today. In some places only the foundations of the ancient wall survive, but recent rescue excavations and extensive cleanups in various parts of its course revealed sections of a height exceeding 4 meters, and gave new information on the course and the construction of the monument. It has mostly been built with blocks of local marble mainly quarried on the hill of the Castle itself.
Particularly characteristic of the oldest phase are the two prominent bulky and oblong blocks on the outer surface of the wall, now visible in the eastern and northwestern parts. The hitherto known mobile finds do not help the dating of the wall but historical factors, data from the excavations of the British Archaeological School at Athens inside the Acropolis and the stylistic characteristics of the masonry can place the beginning of its construction in the late sixth century. BC - a unique example of a marble fortification wall at that time in the Cyclades. With the construction of such a monument, the Sifnians not only sought to protect the Acropolis, but also aimed to promote the wealth of the island. A leading example of this policy is, at the same period, the assignment of the famous Treasure of Sifnos in Delphi.