before fighting ended in Berlin on 2 May 1945, troops of the SS detonated explosives inside the north–south tunnel under the Canal to slow the advance of the Red Army towards Berlin's city centre. The entire tunnel flooded, as well as large sections of the network via connecting passages at the underground station. Many of those seeking shelter in the tunnels were killed. Of the then of subway tunnel, around were flooded with more than one million cubic meters () of water. Demolition and reconstruction (1945–1964) Before a planned reconstruction of the entire could take place, all the war ruins needed to be demolished and cleared away. A popular black market emerged within the ruined area, which the police raided several times a day. One structure demolished after World War II was the 'Rote Burg', a red brick building with round arches, previously used as police and Gestapo headquarters. The huge construction project began in 1886 and was completed in 1890; it was one of Berlin's largest buildings. The 'castle' suffered extensive damage during 1944-45 and was demolished in 1957. The site on the southwest corner of Alexanderplatz remained largely unused as a carpark until the Alexa shopping centre opened in 2007. Reconstruction planning for post-war Berlin gave priority to the dedicated space to accommodate the rapidly growing motor traffic in inner-city thoroughfares. This idea of a traffic-orientated city was already based on considerations and plans by and from the 1930s. East Germany has been subject to redevelopment several times in its history, most recently during the 1960s, when it was turned into a pedestrian zone and enlarged as part of the German Democratic Republic's redevelopment of the city centre. It is surrounded by several notable structures including the (TV Tower). During the Peaceful Revolution of 1989, the demonstration on 4 November 1989 was the largest demonstration in the history of the German Democratic Republic. Protests starting 15 October and peaked on 4 November with an estimated 200,000 participants who called on the government of the ruling Socialist Unity Party of Germany to step down and demanded a free press, the opening of the borders and their right to travel. Speakers were , , , , , and . The protests continued and culminated in the unexpected Fall of the Berlin Wall on 9 November 1989. After German reunification (1989) Ever since German reunification, has undergone a gradual process of