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.
Pillage of Sigtuna
The Pillage of Sigtuna was an event in the summer of 1187 described in the "Eric Chronicles" and in annals from the 13th century. According to descriptions, the town of Sigtuna was burnt on August 12 and Archbishop Johannes of Uppsala was killed near the Almarestäkets fort. At that time, Sigtuna was the most important town of medieval Sweden. The "Eric Chronicles" describes the events as follows:According to the "Eric Chronicles", Jon Jarl was also killed during the incident. The chronicle also describes that these events eventually led to the founding of Stockholm by Birger Jarl, who wanted to protect Sweden from sea invasions.