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 during the Middle Ages
The origin of Stockholm pre-dates its written history and how and why the city evolved in the mid-13th century is still disputed. However, even though events during the 1250s and 1260s remain diffuse, if nothing else, the amount of surviving documents, such as letters and diaries, seem to indicate Stockholm rapidly grew to become the largest city in Sweden during the late 1270s and throughout the 1280s. It is still disputed whether this development was a planned process initiated by the king and German merchants, or a rather spontaneous growth. The lack of the perpendicular city plans and uniformly-sized blocks common in contemporary cities in Germany and Poland, seem to corroborate the latter.