Starting around AD 980, hundreds of workers toiled for years on four impressive structures in Denmark. Circular forts, equipped with ramparts and moats, rose at key locations around the country. They were enormous for the era. The largest, at Aggersborg in North Jutland, measured 240 metres in diameter. As with others of its type, the ring fortress was built using rigorous mathematical principles, with gates placed at the four cardinal points, leading to the four corners of the world. Advanced engineering underpinned the ramparts’ construction, and the streets were covered with oak planks.
The forts were commissioned by the Danish king Harald Bluetooth. They were a show of power intended to awe all who saw them. As the first real king of Denmark, Harald needed to ensure that he commanded…