During the Viking Age, Birka was an important trading center. The archaeological sites of Birka and Hovgården, on the neighbouring island of Adelsö, make up an archaeological complex which illustrates the elaborate trading networks of Viking Scandinavia and their influence on the subsequent history of Europe. Generally regarded as Sweden's oldest town, Birka (along with Hovgården) has been a UNESCO World Heritage Site since 1993.
Established in the middle of the 8th century and thus being one of the earliest urban settlements in Scandinavia, Birka was the Baltic link in the river and portage route through Ladoga and Novgorod to the Byzantine Empire and the Abbasid Califate. Birka was also important as the site of the first known Christian congregation in Sweden, founded in 831 by Saint Ansgar.
Sources are mainly archeological remains. No texts survive from this area, though the written text Vita Ansgari ("The life of Ansgar") by Rimbert (archbishop of Bremen-Hamburg, c. 865) describes the missionary work of Ansgar around 830 at Birka, and Gesta Hammaburgensis Ecclesiae Pontificum (Deeds of Bishops of the Hamburg Church) by Adam of Bremen in 1075 describes the archbishop Unni, who died at Birka in 936. St Ansgar's work was the first attempt to convert the inhabitants from the Norse religion to Christianity, and it was unsuccessful.
Both publications are silent on Birca's size, layout and appearance. Based on Rimbert's account, Birca was significant because it had a port and it was the place for the regional ting. Adam only mentions the port, but otherwise Birca seems to have been significant to him because it had been the bridgehead of Ansgar's Christian mission and because archbishop Unni had been buried there.
Birka was abandoned during the later half of the 10th century. Based on the coin finds, the city seems to have silenced around 960. Roughly around the same time, the near-by settlement of Sigtuna supplanted Birka as the main trading centre in the Mälaren area. The reasons for Birka's decline are disputed. A contributing factor may have been the post-glacial rebound, which lowered the water level of Mälaren changing it from an arm of the sea into a lake and cut Birka off from the nearest (southern) access to the Baltic Sea. The Baltic island of Gotland was also in a better strategic position for Russian-Byzantine trade, and was gaining eminence as a mercantile stronghold. Historian Neil Kent has speculated that the area may have been the victim of an enemy assault.
In search of Birka, National Antiquarian Johan Hadorph was the first to attempt excavations on Björkö in the late 17th century. The later excavations soon indicated that a major settlement had been located on the island and eventually. Ownership of Björkö is today mainly in private hands, and used for farming. The settlement site, however is an archaeological site, and a museum has been built nearby for exhibition of finds, models and reconstructions. It is a popular site to visit during the summer times.References:
The city walls of Avila were built in the 11th century to protect the citizens from the Moors. They have been well maintained throughout the centuries and are now a major tourist attraction as well as a UNESCO World Heritage Site. Visitors can walk around about half of the length of the walls.
The layout of the city is an even quadrilateral with a perimeter of 2,516 m. Its walls, which consist in part of stones already used in earlier constructions, have an average thickness of 3 m. Access to the city is afforded by nine gates of different periods; twin 20 m high towers, linked by a semi-circular arch, flank the oldest ones, Puerta de San Vicente and Puerta del Alcázar.