Trekroner Sea Fortress

Trekroner Sea Fortress is a small, artificial island with two distinctive red barracks, situated at the entrance to Copenhagen Harbour. It was originally part of the Copenhagen Fortifications. Today, visitors have access during summer, allowing them to explore the old gun casements and gloomy, historic cellars, and to enjoy the view of the Copenhagen skyline.

The oldest part of the fortress dates back to the late 18th century. Only after the defeat by the Germans in 1864 was it reinforced with the solid casements, ammunition storage and gun batteries. It was decommissioned as a naval fortress in 1932.

In the years leading up to the Second World War, it was a favourite summer attraction, with a restaurant, cabaret, theatre and other attractions. The buildings remained more or less unchanged, however, and after the war it was left to its own devices. Following extensive renovations, it once more became a popular destination during the summer months.

Today, a café and restaurant are again open on the fortress during summer. The casements now house exhibitions about the history of the fortress and the Copenhagen Fortifications. One room shows a film recounting the 1801 Battle of Copenhagen, in which the fortress played an important role. Visitors are able to wander the dark basement corridors, explore the bastions, relax on the grass slopes or at the café, and enjoy the sun, the view and the fresh sea breeze.

Today, Trekroner is one of the Copenhagen Fortifications’ outreach centres. It has a schools service that organises special teaching activities, and also offers a downloadable smartphone app and game.

Canal Tours sail out to Trekroner. The boat ride through the harbour and across the harbour entrance is a worthwhile experience in itself.

Photo: Scanpix/Arne Magnussen

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  • History
  • Picnics
  • Walks
  • The fortress is open from May to September
  • The fortress played an important role in the Battle of Copenhagen in 1801, preventing Admiral Nelson’s fleet from sailing all the way into the city. Nevertheless, the Danes lost the battle in the end.
  • During the Second World War, the Germans used the fortress as accommodation, and remnants of a mural depicting the Third Reich are still visible.
  • The fortress appears under ‘A’ in Halfdan Rasmussen’s famous ABC for kids:
  • ’Ane lagde anemoner/I kanonen på Trekroner/Ved det allerførste skud/Sprang Anes anemoner ud’ (‘Ane put anemones /In the cannon on Trekroner /At the very first shot/Ane’s anemones popped out’).
  • In spring, many birds nest at the fortress and must not be disturbed.
  • The café on the fortress is open during summer. Picnics can be brought from the mainland.
  • Canal Tours from eg Nyhavn or Langelinie sailing to Trekroner.
1 km

Get directions here