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.
Prehistory and origin of Stockholm
The first, verified, mention of the name 'Stockholm' is from two letters written in Latin in 1252; the first, written in July, is a letter where the King Valdemar and Birger Jarl offered their royal patronage to Fogdö Abbey; and the other, written by Birger Jarl in August, urged the peasantry in Attundaland to pay their tithes to the Uppsala Cathedral. Both letters were written in Stockholm, but give no further information of the city itself or any explanation on the background of the name. However, it can be assumed at least some sort of dwelling in consistence with the station of a Swedish jarl existed at that time.