Karlsvärd Fortress, Fortification in Slite, Sweden.

Karlsvärd Fortress is a pentagonal fortification located on Enholmen island near Slite harbor on the Baltic Sea. The structure consists of stone bastions and defensive walls that rise prominently from the water and dominate the landscape of the small island.

Johan Wärnschiöldh drew a pentagonal bastion sconce for Enholmen in 1650, approved by the King in Council in 1656; two western bastions were begun in 1657, but Charles XI ordered the work closed in 1663. In 1710 the Defense Commission ordered a sconce at Slite, and Anders Sparrfelt's revised plan led to work on Karlsvärd from 1711. On 22 April 1788 the King in Council condemned the unfinished fortress as useless and costly, and Johan Christopher Toll was ordered to blow up the works.

The fortress displays Swedish military architecture from the 18th century, incorporating European fortification techniques visible in its structure today. The pentagonal design and defensive walls shape the island's character and become apparent as you explore the site.

Enholmen lies in the Baltic just off Slite harbour on Gotland's east coast. The ruin has no regular opening hours or guided tours; access depends on private boat or local arrangements. Since 2015 the National Property Board manages the site, and the island was demilitarized in 2011.

In the middle of the 20th century, the old fortress was repurposed as a secret naval weapons control station, giving its military role an unexpected second life. This hidden function remained active until the 1990s and remains completely unknown to most visitors today.