Bourtzi is a tiny peninsula that divides the Skiathos port into two distinct parts. It used to be a fort, which was built by the Gizi brothers who ruled over Skiathos back in 1207. Bourtzi was bordered by impregnable walls and there were two towers located to the left and right of the main gate. Presently, it is not feasible to establish the height of the parapet from the ruins.

In the past Castle used to be one of the principal medieval towns. The town has been strategically built on a rocky cliff that literally hangs above the sea at the northern extreme of the island. The cliff is conspicuous by its enormous height and it was an ideal location for the protection for the people who lived in the Castle town.