Viking ring fortress. Artwork of one of the circular Viking fortresses known as trelleborgs, the remains of which have been found in what is now Denmark and Sweden, dating from the Viking Age (8th to 11th centuries). A strictly circular area was ringed with a defensive wall and divided into four quarters. Each quarter contained Viking longhouses, here arranged in a square. Outside the fortress wall is a defensive moat or trench. This trelleborg is also defended by rivers, with Viking ships at lower left. This layout is based on that found at Slagelse, Zealand, Denmark, thought to have controlled a Viking shipping trading route.