Bourtzi
This lovely small castle-compound on Skiathos harbor stands on a pine-covered peninsula that divides the main port. It was built in 1207 by the Venetian Gyzi brothers to protect Skiathos from pirate attacks. Not much remains of the original fortress (also called the castle of St. George), but a cultural center at the site hosts concerts in the summer, as well as art and antiquities exhibitions. There are plenty of spots to sit and absorb the pretty vistas over the busy waterfront.