Bourtzi is a small peninsula, which separates the port of Skiathos in two. In the past it was a fortress, founded by Gizi brothers who occupied Skiathos in 1207. It was fortified with battlements and turrets and there were two round towers on the right and the left of the gate.

Also, in the fortress there was a water tank and the chapel of St. George, patron of Venice, which was probably built by Gizi brothers. Because of that church, Bourtzi was also called "Kastelli of St. George." The fort was destroyed in 1660 when the island was occupied by the Venetian admiral Francesco Morosini.

In 1823 the islanders managed to push Topal Pasha back. After the liberation of the castle, Bourtzi was turned into the first quarantine of the island. In 1906 an elementary school was built in the center of the peninsula, funded by Andreas Syngrou. At the entrance of the school you can see see the bust of of Alexandros Papadiamantis which has been there since 1925.

Today, Bourtzi is a recreation place, full of pine trees. There are still some remnants of the walls from the old fortification as well as the road leading to the entrance of the old school. The school has been turned into a cultural center with a nice summer theater for theatrical performances.