Oia was given its present name in 1930s. Its previous names were Theraion Polis (city of the Therans), Saint Nikolas Castle and Pano Meria.
Just like the rest Cycladic islands, Oia attracted the interest of the Venetians, who expertised in naval activities, as a fertile place for trading business. So they conquered Santorini and built five castles on it; the Castle of Skaros (Imerovigli) where the Latin Giacomo Barozzi settled with his family and other nobles, the Castle of Pyrgos, the Castle of Emboreion and Saint Nikolas Castle (Ano Meria-Oia).
The Castle of Saint Nikolas was inhabited by Dargenta Family who were descents of Romanos Argyros, a byzantine Emperor. However, Turkish ships captivated the castle in 1577 and the Dargentas were transferred to Syria enslaved.
Piracy was the most horrible danger for the islander during the medieval ages. The greek islands were attacked very often by pirates who plundered the harvests, enslaved men and sold them in the slave markets, while capturing the women for the harems of the Muselims. So, the habitants would either seek refuge to safer island or fortify their settlements against attacks. The Santorinians who had suffered a lot due to the pirates decided to build their housed at the most inaccessible points where it would be hard to be seen by the enemies. They curved their houses or built them very close one to the other with high sheer walls on the side seen by the attackers. In this way, a circuit wall was created by the walls of the houses, which surrounded the castle and the rest of the settlement. So, when reaching Santorini, one would see just a black wall looking like a series of towers.
Small towers had been added along this wall from where the inhabitants could watch the sea and get noticed when a pirate ship was to attack them. Such a tower is known as Goulas (from the Turkish word Koules which means tower). If they had no time to resist the attacker, the Santorians resorted in the castle locking its door or run towards the inland seeking for a safe place. What is more, the villagers used to hide their harvests and their valuable possessions in deep holes they had dug outside the Castle.
The Castle of Saint Nikolas was the most magnificent residence of Santorini. Apart from the house of Dargenta family, there were churches for Christians and Catholics and the houses of the most privileged aristocracy. There were also the Nobles’s Loggia as well as storage places for storing the harvests.
The fortified settlement spread towards Ammoudi as time passed and safety reasons led people to extend the wall to the windmills (used for grinding split peas and barley) in order to protect them.
Nowadays, just a few evidences of this fortification can be seen in Oia as most of it was destroyed due to the earthquake (1956). The ruins of a tower can be found at the northwest edge of the cliff of Oia, known as Goulas area.