Sprogø is a small Danish island located in the center of the Storebælt strait. 

Traces of permanent settlements dating back 4000 years before our time have been found on Sprogø.

In 1170, Valdemar the Great built a fortress on the island to serve as part of a greater defensegrid in an effort to secure the traderoutes on Storebælt against Wendic pirates. The defensegrid also included the construction of large fortresses in both Korsør and Nyborg, which are harbour cities on opposite sides of the strait.

The fortress on Sprogø was only in use for a short time and was torn down after just 100 years. Not many traces remain of the fortress, as many of the building materials were repurposed.

Between 1920 and 1961, the island housed a home for girls. Unstable girls and women of loose morals where placed here in prison-like conditions. All in all, around 500 girls and women were housed on Sprogø.

In 1987, the island was preserved and legally protected. During the construction of the Storebælt bridge, speciel consideration was put on the ruins of Valdemar's fortress and the island's rich fauna. As a result, the area of Sprogø increased fourfold, allowing the bridge and tunnel to connect on the island without causing significant disturbances.