Riddarfjärden
Riddarfjärden (, "The Knight Firth") is the easternmost bay of Lake Mälaren in central Stockholm. Stockholm was founded in 1252 on an island in the stream where Lake Mälaren (from the west) drains into the Baltic Sea (to the east); today the island is called Stadsholmen and constitutes Stockholm's Old Town.
Almarestäket
The strait was of importance before Stockholm was founded in the 14th century, as it guarded the waterway to the significant Swedish medieval cities Uppsala and Sigtuna. For instance, an attack on Sigtuna by Baltic Vikings in 1187 led to a battle at Almarestäket, resulting in the death of Archbishop Johannes.