Trelleborg

The best preserved of the seven known Viking ring castles, built as an exact circle: a rampart five metres high, 17.5 metres wide at the base and 137 metres across, faced with oak, with two roads crossing at right angles in the centre and running out to four gates set opposite each other. Sixteen almost identical longhouses stood four to a quarter. Harald Bluetooth is believed to have ordered it around 980, commanding the Great Belt traffic between Zealand and Funen. It may have housed some 1,300 people. UNESCO listed it with four other ring fortresses in 2023.