Hostile architecture

Hostile architecture is an urban-design strategy that uses elements of the built environment to purposefully guide or restrict behaviour in order to prevent crime and protect property. It often targets people who use or rely on public space more than others, such as youth and the homeless, by restricting the physical behaviours they can engage in. Also known as defensive architecture, hostile design, unpleasant design, exclusionary design, and defensive urban design, the term hostile architecture is often associated with "anti-homeless spikes" – studs embedded in flat surfaces to make sleeping on them uncomfortable and impractical. Other measures include sloped window sills to stop people sitting; benches with armrests positioned to stop people lying on them, and water sprinklers that "intermittently come on but aren't really watering anything." Hostile architecture is also employed to deter skateboarding, littering, loitering, and public urination.
Background.
Although the term "hostile architecture" is recent, the use of civil engineering to achieve social engineering is not: antecedents include 19th century "urine deflectors". Its modern form is derived from the design philosophy Crime Prevention through Environmental Design (CPTED), which aims to prevent crime or protect property through three strategies: natural surveillance, natural access control, and territorial enforcement. 
Consistent with the widespread implementation of defensible space guidelines in the 1970s, most implementations of CPTED as of 2004 are based solely upon the theory that the proper design and effective use of the built environment can reduce crime, reduce the fear of crime, and improve the quality of life. Built environment implementations of CPTED seek to dissuade offenders from committing crimes by manipulating the built environment in which those crimes proceed from or occur. 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. It's important to note that the relationship between welfare, policy and the citizen is complex meaning that the grounds for homelessness are far more difficult to grasp than the simplistic approach to blame towards the operation of the UK’s housing systems.
Applications.
Skateboarding deterrent.
The Camden Borough Council in the UK commissioned concrete-block benches (dubbed "Camden benches") designed to discourage uses such as sleeping, skateboarding and placing stickers.
Public reception.
Proponents say hostile architecture in urban design is necessary to maintain order and safety and deter unwanted behaviors such as loitering, skateboarding and sleeping. Opponents say they target the homeless.
In 2018, British artist Stuart Semple created a social media public awareness campaign encouraging the public to place identifying stickers on instances of hostile design in their environment.