Birger Jarls torn located on terrain feature Riddarholmen
Birger Jarls torn (Swedish for Birger Jarl's Tower) is a defensive tower on the northwest corner of Riddarholmen, an islet in Gamla Stan, the old town of Stockholm. 
The building has been named for Birger Jarl who traditionally is attributed as the founder of Stockholm, but it was built several hundred years later and the name is mostly the product of a 17th-century myth.  Stockholm literally translates to "Log-Islet", and according to that myth the city was founded where a log drifting ashore from Lake Mälar.