During late Iron Age and early Middle Ages, Öland and Möre were right on the borders between Sweden and Denmark. The main fairway went through the straits of Kalmarsund. The Danes were very influential on Öland during the 11th and 12th century.
The church of Resmo, which is one of the largest churches on Öland, was built in a style which shows Danish influence. The church was constructed around 1100. Gråborg became a market-place as well as a centre of the Danish guilds of Knut (see Öland and Trade). A chapel dedicated to this Danish national saint was erected here.
During late Iron Age and the early Middle Ages, the Swedes established their own strongholds in the area. Examples of this are the King´s estate in Hossmo and the fortress Eketorp on Öland. Öland´s most important market-place during early Middle Ages lay at Köpingsvik. It included quite a large area with dwellings, workshops, a harbour and a church. The church at Köpingsvik became the largest parish church on Öland.
All trade was probably controlled by the King, who had a manor house nearby. Towards the end of the 12th century social changes take place and strongholds are established in Kalmar and in Borgholm. Kalmar with its deep harbour now becomes the main port of this region.