Strandpromenaden's hulking centrepiece 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. It was connected to the mainland by a bridge over a moat (filled in during the 19th century) deep enough to accommodate tall ships.