Trekroner Fort (Trekroner Søfort)

Sea gatekeeper at Copenhagen’s harbour mouth

Trekroner Fort, or Trekroner Søfort, sits low and solid on an artificial island where the harbour of Copenhagen meets the Øresund. Long before the current fort rose from the water, three ageing ships of the line were deliberately sunk here in 1713 to form a floating gun platform guarding the narrow approach to the city. One of those ships was named Trekroner – Three Crowns – and the name has followed the fortification ever since. The present masonry and concrete stronghold took shape from the late 1780s, when the Danish crown decided to replace the improvised ship battery with a permanent sea fort. Earth, stone and dredged harbour mud were packed into a timber framework to create a stable island, then ringed with ramparts, bastions and gun emplacements. From the beginning, Trekroner was intended not as a picturesque outpost but as a serious piece of military engineering that could command every vessel sailing toward the capital.

Front-line witness to naval conflict

Even before construction was fully complete, Trekroner found itself in the front line. In 1801 the fort joined Danish ships and floating batteries in the Battle of Copenhagen, firing on the British fleet commanded by Admirals Parker and Nelson. Cannons hastily mounted along its walls helped keep enemy warships at bay, though they could not prevent Denmark’s eventual defeat. Just six years later, in 1807, Trekroner again thundered into action during the British bombardment that set large swathes of Copenhagen ablaze and seized the Danish fleet. Throughout the 19th century the fort was repeatedly strengthened. Casemate blocks were added for protected artillery, underground powder magazines were tucked into the traverses between guns, and a small lighthouse rose above the works to guide friendly vessels safely past the minefields. For decades the stronghold formed part of an integrated defensive ring around Copenhagen, staffed by hundreds of soldiers and bristling with heavy artillery.

From military outpost to island museum

After the First World War, Trekroner’s strategic value faded as technology and geopolitical realities changed. The complex passed to harbour authorities in the 1930s and took on new roles during the German occupation of Denmark, when the island served as barracks and later as a temporary prison for German officers. In the quieter post-war decades the fort slipped into disuse, its walls weathered by salt and wind, until a careful restoration opened it to the public in the 1980s as a historic site. Today you step into a place where traces of every era still overlap. Heavy, rusting gun mountings sit beside modern safety railings, while exhibition rooms in former gun casements tell stories of sea battles, coastal defense and life in an isolated garrison. Simple digital installations and displays help you piece together how this low artificial island once stood at the centre of European power struggles.

Exploring casemates, ramparts and shoreline

A visit unfolds at your own pace. From the landing pier you fan out across grassy embankments and gravel paths that loop around the island’s perimeter. The heart of the fort is its multi-storey casemate building, where arched brick chambers once sheltered crews, ammunition and artillery. Upstairs, open emplacements reveal how cannons were spaced and protected; below, cool, dim passages cut through the interior like a honeycomb, some now housing small exhibits, others left deliberately raw. Outside, low parapets frame wide views back to Copenhagen’s skyline – church spires and modern towers rising beyond the harbour – while the constant movement of ferries and cargo ships reminds you that this shipping lane is still very much alive. On sunny days, visitors sprawl on the grassy slopes with a picnic, using the old earthworks as informal seating. In summer seasons, a small café sometimes operates in one of the buildings, adding coffee and ice cream to the mix of cannons and coastal scenery.

An atmospheric escape just offshore

Part of Trekroner’s charm lies in the journey required to get here. The fort is accessible only by boat, so the short crossing from central Copenhagen doubles as a miniature harbour cruise. As you approach, the thick walls seem to rise straight from the water, with the Danish flag usually snapping in the wind above the ramparts. Once ashore, the city’s noise falls away, replaced by gull calls, the slap of waves against stone and the occasional thud of boots on old wooden floors. The island is compact enough to explore in an hour or two, yet layered enough to reward lingering. History enthusiasts pore over gun positions and wartime photographs, while families weave in and out of tunnels with torches in hand. Photographers seek out silhouettes of cannons against the sky and long perspectives along the outer wall. Whether you treat Trekroner as a half-day excursion or a brief detour on a wider waterfront itinerary, it offers a rare chance to stand in the exact channel where Copenhagen has been defended for more than three centuries.