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 in diameter.
According to a 1989 archeological survey, the structure was built in the 1130s and may have been used for less than a century. Although its original purpose is unknown, theories suggest it was either used as a shelter during the turbulent times on Gotland at the end of the Viking Age or that it was the site of a last stand.
Among the archeological finds at the Bulverket are the remains of three boats. One of these served as a model for the reconstruction of a Viking boat, the Krampmacken, in 1980. Krampmacken has subsequently made several journeys following old Viking waterways through Eastern Europe.
Etymology
The name Bulverket originates from the old Swedish words bul, meaning "log", and verk which refers to something "built". It is related to the English word "bulwark", a form of fortification, as well as the Dutch bolwerk with the same meaning and the Swedish bålverk.
Discovery
It is likely that the Bulverket has been known to the local people for centuries, as thousands of logs can be seen through the water on a calm day, and it is also considered the best place for perch fishing on the lake. However, there is no oral lore regarding its construction, purpose, or fate. All that remains is a saying used in the northern part of Gotland that concerns large amounts of smoke: "There is smoke just like when Lake Tingstäde burned".()
The Bulverket was first mentioned in written sources in 1868, when Swedish archeologist Oscar Montelius wrote that naturalist and teacher Dr. Lindström discovered poles in Lake Tingstäde in 1866.
One of the recommended ways of seeing the Bulverket is during the winter, when the ice is smooth and clear and the lake is used for tour skating.
Construction
The Bulverket is the remnants of a large wooden fortification or bulwark on the island of Gotland. It was raised in the middle of Lake Tingstäde, in the least accessible part of the lake. It has now collapsed and the logs are scattered over an area of . The main part of the Bulverket consisted of four piers on square caissons made of logs. The piers, wide, formed a square platform, each side of which was , with an open, but sheltered, water area in the middle. The platform was aligned so that each side faced one of the cardinal directions. On the platform were houses for people living on the Bulverket as well as storage sheds, and in the central part were landings for mooring boats. It is estimated that about 200 ships of the early Viking Age type could be moored there.
Platform and palisade
The platform was enclosed by two concentric circles of poles (or timber piles) driven into the bottom of the lake. In some parts, this outer defence stood as far as from the platform. The entire construction was about in diameter. The palisade and the platform had openings in the northwest part of the construction to allow boats in and out. Analysis of vertical posts show that the water surface is at approximately the same height when the Bulverket was built as it is today, but that the depth of the water has decreased due to sedimentation. The sedimentation layer is on the site.
The Bulverket was not surveyed and built as a single unit, but rather one caisson at a time. Some of these caissons were linked by making rectangular holes in the ends of the vertical logs, stacking the logs on top of each other and driving a horizontal pole about through the holes. The caissons were approximately high and covered with a flooring that rested above the water.
Houses
The houses were built using different methods such as post-and-plank, log house technique and palisade walls. Construction details found at the site indicates that the houses were approximately . A total of about 25,000 logs, mostly pine, were used to build the Bulverket. This equals approximately of forest. Studies of the construction show that it was built in a relatively short period of time, perhaps no less than a year. This would have required about 100 men to work on the site. Given the amount of organization needed for such a project and that the farms around Lake Tingstäde could not spare all of their workers for the construction, most of northern Gotland must have been involved in the work.