"Thanks to high-resolution geophysical surveys carried out in September 2016, researchers now believe they have located one of the most important Viking halls of the era, situated in the harbour bay of Korshamn (1) outside of Birka's town boundaries. They believe that it can be dated to the period after 810 AD." Source: The Local.
(1) I'm not sure if I've linked to the right Korshamn. The geography here is kind of confusing.
Note: "The Birka archaeological site is located on Björkö Island in Lake Mälar and was occupied in the 9th and 10th centuries," according to the UNESCO World Heritage List. Video below shows a digital reconstruction of the Viking town of Birka.
It's hard to find detailed information about Birka in English. The next documentary is captioned and has a lot of good maps and views of the region.
"Established in the middle of the 8th century, Birka was the Baltic link in the river and portage route to the Byzantine Empire," according to The Local. "Birka was also important as the site of the first known Christian congregation in Sweden, founded in 831 by Saint Ansgar."
According to A History of the Vikings by Gwyn Jones (p. 168), Birka was a major Viking trade hub and market town well-situated for defense:
"Security from enemies and accessibility to friends were considerations much in the minds of those who founded the trading town of Birka on the island of Bjorko in Lake Malar. The incoming merchant, having traversed the thirty labyrinthine miles of islands and skerries east of Stockholm, must complete a further eighteen miles of observed navigation through the island-studded lake before reaching Bjorko. The island is so lonely and quiet under its birch glades today, its remains so mouldered in grassy earth, that it requires a strong effort of the imagination to see it as it was, a hub of traffic whose spokes reached out to England and Frisia in the west, Lake Ladoga and the Middle Volga in the east, Uppsala and the fur-bearing lands in the north, Gotland, Truso, Wollin, Hedeby, and all that lay beyond them, to the south."