of use, and its design was forgotten. Middle Ages A transition from galley to sailing vessels as the most common types of warships began in the High Middle Ages (c. 11th century). Large high-sided sailing ships had always been formidable obstacles for galleys. To low-freeboard oared vessels, the bulkier sailing ships, the cog and the carrack, were almost like floating fortresses, being difficult to board and even harder to capture. Galleys remained useful as warships throughout the entire Middle Ages because of their maneuverability. Sailing ships of the time had only one mast, usually with just a single, large square sail. This made them cumbersome to steer and it was virtually impossible to sail into the wind direction. Galleys therefore were still the only ship type capable of coastal raiding and amphibious landings, both key elements of medieval warfare. Eastern Mediterranean In the eastern Mediterranean, the Byzantine Empire struggled with the incursion from invading Muslim Arabs from the 7th century, leading to fierce competition, a buildup of fleet, and war galleys of increasing size. Soon after conquering Egypt and the Levant, the Arab rulers built ships highly similar to Byzantine dromons with the help of local Coptic shipwrights from former Byzantine naval bases. By the 9th century, the struggle between the Byzantines and Arabs had turned the Eastern Mediterranean into a no-man's land for merchant activity. In the 820s Crete was captured by Andalusian Muslims displaced by a failed revolt against the Emirate of Cordoba, turning the island into a base for (galley) attacks on Christian shipping until the island was recaptured by the Byzantines in 960. Western Mediterranean In the western Mediterranean and Atlantic, the division of the Carolingian Empire in the late 9th century brought on a period of instability, meaning increased piracy and raiding in the Mediterranean, particularly by newly arrived Muslim invaders. The situation was worsened by raiding Scandinavian Vikings who used longships, vessels that in many ways were very close to galleys in design and functionality and also employed similar tactics. To counter the threat, local rulers began to build large oared vessels, some with up to 30 pairs of oars, that were larger, faster, and with higher sides than Viking ships. Scandinavian expansion, including incursions into the Mediterranean and attacks on both Muslim Iberia and even Constantinople itself, subsided by the mid-11th century. By this time, greater stability in merchant traffic was achieved by