During the Middle Ages, trade in the Baltic flourished under the auspices of the German Hanseatic League. The Danish itinerary from the 13th century, describes a route that stretched from the Swedish coast eastwards through the Åland islands and Kökar to Aspöskären. From there, the route continued to Nötö, Hitis aroung the Hangö peninsula and further east along the coast of Nyland.
Important waterways were marked with raised stone cairns on high points of adjacent islands. High landmarks were also important for medieval navigators. Anchored tar barrels were used to mark narrow entrances to ports.
Nagu Gyttja served as shelter port for merchant ships on their way to Turku. Ships enroute from Åbo to Nyland followed a straight route south to the Pargas strait and Tervsund, Pargas Port and to Junfrusund and onward. As the Pargas strait became too shallow the ships had to sail around through Hessund.
Piloting of ships was an important source of income for people in the outer archipelago. In 1571, the pilots received five box measures (kappa) of rye for each week they were on a pilot trip. Autumn storms could surprise even experienced skippers and many accidents took place in the treacherous archipelago. A famous medieval wreck is the cargo ship that sank at Egelskär in Nagu around the year 1300. The ship is likely to have been on its way to Turku and was loaded with German stoneware, iron, Danish limestone, grindstones from Norway and a church bell. Two other medieval wrecks, both about 17 meters long and built in oak, have also been discovered along this route. One wreck is found at Gråharun north of Utö and the other at Metskär near Rosala.
