The Kathisma (meaning "seat") of Agios Vassilios used to be an -almost- independent monastery with the name Paliomonastiro. It was founded in the 9th century by the monk Vassilios. The complex took its current form in the beginning of the 14th century.
It is a dependency of the Monastery of Chilandarion which since 1198 was granted by the Byzantine emperor to the Serbs.

It is on a rock by the sea, west of the arsanas (port) of the Chelandarion. It has distinct fortification features and was protected by a moan, over which a movable bridge led to the main gate.

The oldest building here is the main tower, a typical late-Byzantine tower with strong buttresses on the sides. It is 15m high and its base dimensions are 11.0x13.5m. On the top, there was a small chapel that does not exist anymore.
The tower was built in 1301-1302 by the Serbian king Stefan Milutin. Later, in 1330, his son built the church of Agios Vassilios which gave its name to the complex.

The Kathisma of Agios Vasilios sometimes is referred as Fort Of Chrysi. "Chrysi" was the name of an ancient Greek city in the area.