In the 9th century, 6 viking ships was deliberately sunk in Roskilde fjord, to improve the city's defense against attack from the sea.
In the 1960'ies the ships was raised and the museum was build.
Next to the museum is a museum shipyard that is building replica viking ships with the same methods as was used a thousand years ago.
World class attraction.
Build after the wars with Germany in 1848 and 1864, as a very long defence line around Copenhagen.
Actually, this was the largest construction project in Denmark ever, only outperformed by the Great Belt Bridge a hundred years later.
Modern guns made the fortification obsolete in less than 30 years.