context: Traditionally, Stockholm was founded by the Earl of Birger, who wanted to prevent enemies from entering the Baltic Sea into Lake Mälaren. Indeed, the first buildings in Stockholm were forts designed to restrict traffic between these waterways. Stockholm was mentioned in historical documents in 1252 and at the same time it was granted city rights. King Valdemar Birger's son signed a trade agreement with the Hanseatic League in Stockholm. Archaeological excavations show that Stockholm was inhabited as early as the 11th century. In the 12th century a fortress was built on the hill of the present royal castle. In the 1270s the Franciscans established a monastery on what is now Riddarholmen. [1]
question: Who founded Stockholm?
answers.text: Earl of Birger