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
As ice lightened its grip of the area about 11,500 BCE, the area was inundated by melt water before the land started to rise and the first islets rose over the water surface (at the time located about 40 metres over the present sea level). The retiring ice left behind a cover of sand, gravel, and rocks forming moraine ridges still witnessing how the ice gave up some 250 m annually over two centuries. Under the ice sheet, streams formed eskers, most notably the huge Brunkebergsåsen, the steep slopes of which still form barriers in central Stockholm.