answers: August 1968
context: World Trade Center, most centrally its twin towers, involved many other innovative techniques, such as the slurry wall for digging the foundation, and wind tunnel experiments. Construction of the World Trade Center's North Tower began in August 1968, and the South Tower in 1969. Extensive use of prefabricated components helped to speed up the construction process. The first tenants moved into the North Tower in December 1970 and into the South Tower in January 1972. Four other low-level buildings were constructed as part of the World Trade Center in the early 1970s, and the complex was mostly complete by 1973. </s> tunnel tolls to raise funds, with a 56 percent toll increase scheduled between 2011 and 2015; however, the proceeds of these increases were not used to pay for the tower's construction. Many of Daniel Libeskind's original concepts from the 2002 competition were discarded from the tower's final design. One World Trade Center's final design consisted of simple symmetries and a more traditional profile, intended to compare with selected elements of the contemporary New York skyline. The tower's central spire draws from previous buildings, such as the Empire State Building and the Chrysler Building. It also visually resembles the original Twin </s> two years. In 2006, the Port Authority took over from Silverstein Properties. The project's developer Tishman Construction Corporation was the construction manager at the time. Construction began in April 2006. Digging the foundation and installing tower-foundation steel columns, concrete, and rebar took twice as long as it normally would due to the existence of the subway line under West Broadway nearby. The building reached grade level by 2010, progressed at a pace of one floor a week, topped out in August 2012, and was structurally completed in May 2013. The building opened on November 3, 2014, and the first 170
question: when did they start building the twin towers