Suomenlinna ("Sveaborg", "Viapori") sea fortress is a UNESCO World Heritage Site and one of Finland’s most popular tourist attractions. The construction of the fortress started by the king of Sweden in 1748 as protection against Russian expansionism. Suomenlinna was planned to be a principal base for naval military operations and the general responsibility for the fortification work was given to Augustin Ehrensvärd. The construction phase lasted for over four decades. During this time six islands were fortified to provide a safe harbor and dock for the archipelago fleet in Finland.
In the Finnish War (1808-1809) Russians easily took Helsinki in early 1808 and began bombarding the fortress. Its commander, Carl Olof Cronstedt, negotiated a cease-fire, and when no Swedish reinforcements had arrived by May, Suomenlinna surrendered with almost 7,000 men. After taking over the fortress the Russians set about on an extensive building program, mostly extra barracks, but also extending the dockyard and reinforcing to the fortification lines.
During the Crimean War in 1855 a combined Anglo-French fleet bombarded Viapori for two days in August. At this point, the repair work was nowhere near complete, and Viapori sustained heavy damage in the bombardment. The next stage in the arming of Suomenlinna and the Gulf of Finland came in the build-up to World War I. The fortress and its surrounding islands became part of "Peter the Great's naval fortification" designed to safeguard the capital, Saint Petersburg. The fortress became part of an independent Finland in 1917, following the Russian Revolution.
After the Finnish Civil War, a prison camp existed on the island. About 6000 red prisoners were held in Suomenlinna. Many of them were executed by the white army and others died of disease due to the poor conditions in the fortress.
Suomenlinna is today one of the most popular tourist attractions in Helsinki as well as a popular picnicking spot for the city's inhabitants. A number of museums exist on the island, as well as the last surviving Finnish submarine, Vesikko.
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.