The most prominent feature that sits along the Strandepromenaden is the distinctive Christiansholm Fortress. Built by royal decree between 1662 and 1672 to keep watch over the strategic Skagerrak Straits and protect the city from pirates and rambunctious Swedes, the construction featured walls up to 5m thick and an armoury buried within a concentric inner wall, all of which came at a price: 1550 local citizens were taxed to fund the project and coerced into labour. It was connected to the mainland by a bridge over a moat (filled in during the 19th century) deep enough to accommodate tall ships. The fortress served its purpose – it was never taken by enemy forces. The original roof was destroyed by fire in 1872 and a new roof with glass clerestory windows was built. A ring of eight bronze cannons, cast between 1666 and 1788, still menaces the offshore skerries. Free guided tours run on Sundays from mid-June to mid-August.