Bulverket

The Bulverket is the remnants of a large wooden fortification or bulwark at Lake Tingstäde on the island of Gotland, Sweden. When built, it consisted of a platform with houses surrounded by a double palisade with the entire construction around 250 m (820 ft) in diameter.

Source: Wikipedia (CC BY-SA 4.0) — https://en.wikipedia.org/wiki/Bulverket