The island has roughly rhomboid shape, elongated in southwest-northeast direction. The longer diagonal is about 12 km long, and the island is about 6 km wide in average. The coast is interspersed with numerous inlets, capes and peninsulas.
The terrain is hilly, more rugged on north, and the highest point is at the altitude of 433 m. Southern and southeast parts have gentler slopes, and most of settlements and facilities are located there.
The main road runs along southern and eastern coast of the island, with several smaller and dirt roads reaching the inland and northern coast. The town of Skiathos and the airport are located at the northern part of the island’s eastern side. Tsougria and all other main islets are located a few kilometres off the eastern coast, and are visible from the town.[

Much of the island is forested. The area around the villages and the town are farmland. The island’s forests are pine and are concentrated in the southeastern part of the island. The island of Skopelos can be seen from Skiathos with the more distant islands of Euboea and Skyros also visible in clearer conditio

History

In Ancient times, the island played a minor role during the Persian Wars. In 480 BC, the fleet of the Persian king Xerxes was hit by a storm and was badly damaged on the rocks of the Skiathos coast. Following this the Greek fleet blockaded the adjacent seas to prevent naval invasion and provisions for the enemy of 300 Spartans who stood heroicaly at Thermopylae pass. Persian fleet was defeated there Artemisium and finally destroyed at the Battle of Salamis a year later.

Skiathos remained in the Delian League until it lost its independence.
The city was destroyed by Philip V of Macedon in 200 BC. In 1207 the Gyzi brothers captured the island and built the Bourtzi, a small Venetian-styled fortress similar to the Bourtzi in Nafplio, on an islet just out of Skiathos town, to protect the capital from the pirates.
But the Bourtzi was ineffective in protecting the population and in the middle 14th century the inhabitants moved the capital from the ancient site that lay where modern Skiathos town is to Kastro (the Greek word for castle), located on a high rock, overlooking a steep cliff above the sea at the northernmost part of the island. In 1704 monks from Athos built the Evangelistria monastery which played a part on the Greek War of Independence as a hide-out for Greek rebels.

The first flag of Greece was created and hoisted in the Evangelistria monastery in Skiathos in 1807.
Several prominent military leaders (including Theodoros Kolokotronis and Andreas Miaoulis) had gathered there for consultation concerning an uprising, and they were sworn to this flag by the local bishop.

After the War of Independence and demise of piracy in the Aegean, Kastro became less important as a strategic location. In 1830s, the island’s capital was relocated to the original site — where it still remains.

Today, ruins of Kastro are one of tourist attractions. During the 19th century Skiathos became an important shipbuilding centre in the Aegean due to the abundance of pine forests on the island.

The pine woods of the island were then almost obliterated. This was brought to a halt though, due to the emergence of steamboats. A small shipwright remains north of Skiathos town, which still builds traditional Greek caiques. The film Mamma Mia was partially filmed on Skiathos and nearby island Skopelos.