The strikingly beautiful Riddarholmskyrkan, on the equally pretty and under-visited islet of Riddarholmen, was built by Franciscan monks in the late 13th century. It has been the royal necropolis since the burial of Magnus Ladulås in 1290, and is home to the armorial glory of the Seraphim knightly order. There’s a guided tour in English at noon (included with admission) and occasional concerts. Holiday closures are frequent; check the website for updates. Admission fee is by credit card only.
Wall plates display the coats of arms of the knights. Look for the marble sarcophagus of Gustav II Adolf, Sweden’s mightiest monarch, who died in 1632. Riddarholmen is easy to reach by footbridge from the Gamla Stan tunnelbana station.