The round tower with a 16th-century secret

The Birger Jarl's Tower, located on the northwest corner of Riddarholmen in Stockholm, is one of the oldest buildings in the town. Officially known as Birger Jarl's Tower, it was built by King Gustav I in 1530. The tower, standing 19.5 metres high, was actually designed to replace timbered redoubts destroyed by a fire in 1525. The timber used for construction was taken from St. Clare's Priory and churches around the city. The tower has undergone several changes over the centuries, but the most significant alterations took place in 1589 when the original crenellation was replaced with a third floor and topped with a cone-shaped roof. Although it has had various uses throughout its history, it now houses the offices of the Chancellor of Justice.

Sources: Wikipedia, OpenStreetMap, Wikidata, Wikimedia Commons