London built the Ring of Steel—a network of concrete barriers, checkpoints, and thousands of video cameras around the historic City of London—after bombings by the Irish Republican Army. The idea was to monitor everyone entering and leaving the Square Mile, what the The New York Times later called “fortress urbanism.” After the September 11, 2001, attacks, city planners looking to defend New York from terrorism turned to London and fortress urbanism for inspiration.

Source: Wired – Threat Level