Hostile architecture is an idea from Urban design architecture should be used to guide people towards wanted behaviour. The opposite can also be true: Design in such a way that certain unwanted behaviour becomes uncomfortable, or is not possible at all any more. Hostile architecture often targets people who use public spaces more than others: Young people, poor people or homeless people. It does this by restricting the physical behaviours they can engage in.^[1]

Other names for the concept are defensive architecture, hostile design, unpleasant design, exclusionary design, and defensive urban design. The term hostile architecture is often associated with items like "anti-homeless spikes". These are studs in flat surfaces to make sleeping on them uncomfortable and impractical. This form of architecture is most common in densely populated and urban areas.^[2][3] Other measures include sloped window sills to stop people sitting; benches with armrests positioned to stop people lying on them; water sprinklers that spray from time to time; and public trash bins with inconveniently small mouths to prevent the insertion of bulky wastes.^[4] Hostile architecture is also used against skateboarding, BMXing, inline skating, littering, loitering, public urination,^[5] and trespassing, and as a form of pest control.^[6]

The term "hostile architecture" is quite new. This is not the case for the use of civil engineering to achieve social engineering: Examples are urine deflectors, desgned in the 19th century, or urban planning in the United States that takes the idea of racial segregation into account.^[7][8][9] American urban planner Robert Moses designed a stretch of Long Island Southern State Parkway with low stone bridges so that buses could not pass under them. This made it more difficult for people who relied on public transportation, mainly African Americans, to visit the beach that wealthier car-owners could visit.^[10] Outside of the United States, public space design change for the purpose of social control also has historic precedent: the narrow streets of 19th century Paris, France were made wider for the purpose of allowing the military easier ability to stop protests.^[11]

In its modern form, the design philiophy crime prevention through environmental design, or CPTED, influenced it. This philisophy uses three strategies to prevent crime, and protect property: Natural surveillance, natural acess cotrol, and territorial enforcement.^[12] Excusionary design is becoming more common, especially in large cities, such as Stockholm.^[13][14][15]

Such guidelines have often been implemented, starting in the 1970s. As of 2004, most implementations of CPTED rely on the idea that if the built environment was properly designed, and used effectively, this could reduce crime, and the fear of crime. With this, the quality of life would be improved. When CPTED is implemented, the environment where the crimes would occur is changed, so that committing the crimes becomes more difficult.

The six main concepts according to Moffat are territoriality, surveillance, access control, image/maintenance, activity support and target hardening. Applying all of these strategies is key when trying to prevent crime in any neighborhood, crime-ridden or not.^[16]

Beyond CPTED, scholarly research has also found that modern capitalist cities have a vested interest in eliminating signs of homelessness, fearing that it might discourage investment from wealthier people.^[17] In England, much of their hostile architecture has been attributed to a desire by the government to fight an anti-social street scene, taking the form of begging and street drinking.^[18]

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  Boulders installed along a freeway ramp in Portland, Oregon, United States as a hostile architecture to deter transient camps.
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  The "Camden bench", used in London, has a design that is stated to discourage sleeping, littering, skateboarding, drug dealing, graffiti and theft
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  Benches with metal pipes at a train station in Vienna.
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  Anti-homeless spikes in New York, designed to prevent sitting
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  Anti-homeless object in Shinjuku Station underpass.
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  Fence under the stairs of the City Archives in Kungsholmen in Stockholm (2015).^[19]