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, with more outside the fort, and bridges crossing defensive moats and trenches. This layout is based on that found at Slagelse, Zealand, Denmark, thought to have controlled a Viking shipping trading route, with ships (on the river) navigating a lake to reach the sea.