The ancient fortress Ismantorp, on the island of Öland, perplexes researchers. With its many gates and evidence of intense inhabitation, it differs from other fortresses on the island. Ismantorp, which today is surrounded by marshes in the Mittlandskogen forest, is an archaeological enigma as well as a popular place to visit.

The first written description of Ismantorp dates back to the year 1634. Renowned botanist Carl von Linné, as well as other travellers and scholars, paid attention to the fortress. Researchers have tried to determine the fortress's age, function and history since the 19th century. Its well-preserved walls distinguish the fortress, as do the central building's unusual location on a ground elevation, the 88 visible house foundations and the nine gates.

Houses were built on practically all space within the fortress-walls. Ruins of large and small houses can be seen along a network of streets and alleys that bind all parts of the fortress to the gates in the fortified wall. There is a square in the middle of the fortress area, and the town planning is reminiscent of a medieval community.

Theories of Ismantorp's function differ widely. The fortifications suggest a defensive purpose, but its many gates make is difficult to defend. Ismantorp fortress has been compared to large Slavic castles that were both protective fortress and religious sanctuaries. Other researchers view the fortress as an integrated part of a united defence of the island of Öland.

Limited archaeological excavations were carried out in the fortress building during the summer of 2000. A number of ancient items were uncovered, for example an arrowhead and an iron clasp for clothing. Hitherto, the findings indicate that the fortress was used between the years 300-500 A.D. The excavations continued in the summer of 2001 to gain further insight into the meaning of the artefacts.