Prehistory and origin of Stockholm
According to the Chronicle of Eric, written in the 1320s, Stockholm was founded by Birger Jarl around 1250 as a lock to the Lake Mälaren region ("laas fore then sio", "lock before that lake") in order to prevent pirates from reaching the seven cities and nineteen parishes around it. Another medieval source ("Visbyannalerna"), however, claims the city was founded in 1187 following a pagan pillaged the city of Sigtuna, and as there was an "Earl" named Birger around at that time too, the disputed origin of the city are likely to remain obscure and some historians choose a diplomatic interpretation saying there was some sort of fortification around by the mouth of lake Mälaren when the city was founded during the second half of the 13th century. As Snorri mentions no city in his account but some sort of fortification called a "kastali" (in various manuscripts curiously said to be located east and west of "Stocksund"), it has generally been assumed this fortification eventually developed into the castle Tre Kronor located where still is the Stockholm Palace.
Stockholm
Stockholm's location appears in Norse sagas as Agnafit, and in Heimskringla in connection with the legendary king Agne. The earliest written mention of the name Stockholm dates from 1252, by which time the mines in Bergslagen made it an important site in the iron trade. The first part of the name ("stock") means log in Swedish, although it may also be connected to an old German word ("Stock") meaning fortification. The second part of the name ("holm") means islet, and is thought to refer to the islet Helgeandsholmen in central Stockholm. According to Eric Chronicles the city is said to have been founded by Birger Jarl to protect Sweden from sea invasions made by Karelians after the pillage of Sigtuna on Lake Mälaren in the summer of 1187.