Västervik was the commercial centre of Tjust during the Middle Ages. As early as in the Viking Age, there was a market-place by the Gamleby bay (Gamlebyviken). The town of Västervik was established by the King during the first part of the 13th century. The town was rather small, with a harbour, a square, a town hall and a church, the Church of St. Gertrud.
Västervik kept in touch with other towns on the Baltic Sea. One imported goods from other Swedish towns, as well as from the towns in northern Germany. From Lübeck and Danzig, one got salt, cloth, hop, beer, wine, spices and so on. The peasants of Tjust would travel to town to buy and sell. There would probably be fairs on a regular basis. From Västervik, the produce of the peasants would be exported; butter, cheese, meat, horses and oxen.
The Gamleby Bay became increasingly shallow during the Middle Ages. King Erik of Pomerania decided that the town should be moved. During the years 1429-1433, Västervik was established were the Castle of Stäkeholm once lay. New Västervik was then complete with new houses, streets, harbour, square, church (St. Gertrud ´s) and Town Hall. The old town was called Old Västervik, later Gamleby. Västervik has been burned down several times, during attacks from enemy armies. This happened in 1452, 1517, 1612 and 1677.
The main thoroughfare for merchant ships went through the Tjust archipelago. Ships from North Germany, sailing to Stockholm or Reval (Tallinn), passed here. Beacons and cairns showed the way.