Written by Andrea Lo, CNN

It looks like something designed to stop inmates from escaping a prison complex. But this shocking photograph of metal spikes covering the branches of a tree was actually taken in a wealthy residential suburb of Bristol, England. Their purpose? To prevent birds from perching on the branches, and defecating on the cars below.

Despite outrage surrounding the spikes exploding on social media this week, they will remain. "There's nothing the council can do because it's on private property and there's no tree protection order in place," a spokesperson for Bristol City Council told CNN.

































1 / 17 – London, United Kingdom Hostile architecture is a form of urban design that aims to prevent people from lingering in public spaces. The anti-homeless spikes here, for example, were installed to deter beggars and those sleeping rough. Credit: amer ghazzal/Alamy Live News

This installation is a prime example of hostile architecture -- a controversial type of urban design usually aimed at preventing people from using public spaces in undesirable ways. It can come in the form of spiked or sloped benches, bolts installed on shop doorsteps and windowsills, and even water features that operate at surprising intervals on flat surfaces.

While use of design to block homeless people from finding a place to sleep or young people from skating in public angers many people, others feel it's a necessary measure to secure public safety.

Dean Harvey (L) and James Furzer (R). Credit: Dean Harvey, James Furzer

CNN invited architect James Furzer, whose designs try to combat hostile architecture, to debate this issue with Dean Harvey, co-founder of Factory Furniture: a company that produces many of the offending "hostile" benches.

Here's how their Skype conversation went.

You've both seen the shocking tree spikes in Bristol. What is your reaction?

James Furzer: This is totally hostile architecture. Hostile architecture doesn't affect humans only -- the landscapes we design can also be manipulated to deter all sorts of acts. It's such a shame that it now applies to nature.

Not only does it look unattractive and aggressive, it shows the selfish nature of society and its thoughts on the environment. Has anyone considered who takes the blame if children wish to playfully climb the trees, unaware of the barbaric devices situated on top of the branches? (Or) if a branch falls, with an unsuspecting person relaxing beneath?

Dean Harvey: My initial thought is that this cannot be considered an extension of hostile architecture: spikes have been used to protect damage to stonework from bird droppings for many years.

This is an extension of pest control. I think the issue here is the damage to the ecosystem of the tree and the aesthetic problem it poses. It's totally unnecessary. A tree is a natural object, and this (the addition) is a complete eyesore. The time and effort they spent putting those spikes on trees is probably totally outrageous in comparison to any actual problem.

How do you define hostile architecture in general?

DH: Hostile architecture is where architectural elements and the public realm are used to control human behavior.

JF: I'd agree with that.

How does it influence people's behavior?

DH: It can provide a solution: prevent drug drops, minimize the amount of time people spend in an area. With a perched or sloped surface, people can't loiter for too long.

JF: I think (whether or not those influences are positive), comes down to what you would classify as antisocial behavior, really. Drug use is a different kind of antisocial behavior to skateboarding. It's a criminal activity that has a negative impact. Sleeping rough and hanging out with a group of friends isn't particularly criminal.

Is it really a bad thing that you're encouraging people to hang around those spaces? Is that not what architecture and design are about? If we designed a building where people didn't want to stay for too long, because it's hostile and uncomfortable, have we succeeded in our jobs as architects? I don't think so.

Toronto-based researcher Cara Chellew runs website #defensiveTO, documenting photos of hostile architecture. Credit: #defensiveTO

DH: You need to be able to walk out of your house and feel safe. (However), when you consider a bunch of kids hanging out on the street corner antisocial behavior, you're in very dangerous territory. Many councils (in the UK) consider that unacceptable. We were all kids once. And I think, to stop our children socializing is dangerous.

So when did this type of architecture start to emerge and why?

DH: It's been around for centuries. If you look at nice Georgian-era buildings (in the UK), there are spikes on walls and fences. They were originally intended to deter, or harm, someone attempting to scale a wall or an iron fence.

Anti-loitering spikes on a window edge in Paris, France. Credit: Cara Chellew

JF: It's been around for hundreds of years, but it's only recently gained publicity and interest. With the rise in homelessness, it becomes more noticeable when you see spikes in areas that people would be sleeping rough in. Homeless people are geniuses -- they will find shelter wherever they can. It's only after that that people realize, "Hold on, they're using that to sleep rough, we need to stop this." Then the add-ons appear.

DH: We've been asked to add studs to benches and things like that. In the end, I suggested that they put in an armrest, which has a dual use. The whole idea of an add-on is just poor design at the concept stage.

What are the pros and cons of hostile architecture?

JF: I'm generally against hostile architecture, but I understand it has a place in society. There are designs and theories that actually encourage self-policing and self-security within different town layouts -- something that is seen in every town we inhabit. That in itself is a form of hostile architecture. I understand its requirements to an extent, but I do feel that more recently ... we are designing people potentially out of the spaces. It's become aggressive.

DH: When we design a piece of furniture, we're not thinking about it being antisocial -- we're thinking about that piece being used as a piece of furniture in the public realm, rather than as a skating pit, or for grinding an object, or as a hangout (area).

When we designed the Serpentine bench in the 1990s, one of the things people mentioned was that it couldn't be slept on. We then used that as one of our marketing points -- that it could deter rough sleepers, and skaters.

The Serpentine bench. Credit: Factory Furniture

But isn't hostile architecture treating the symptom not the problem?

DH: When we designed the Camden bench, we were given an extensive list of requirements, on a fairly small budget. We didn't have time to address why any of those problems existed. We just came at it from a fairly blunt angle. (The council required that people) couldn't sleep on it, stash drugs in it, or skate on it. When you level all those things up, it comes out as a pretty defensive piece of furniture, but in fact, all we're doing is enabling it to be used as a piece of furniture -- so people could walk into town and take a rest on it.

The Camden bench, first installed in London in 2009. Credit: Factory Furniture

Look at the underlying problem Camden had: They were taking out benches because of antisocial behavior. There was a lot of pressure from local residents to not install anymore benches, because public areas can become hangouts. It's a problem -- you either put in no seats, at the request of the residents, or you come up with a design that prevents long stays and day drinking.

We need to design spaces that encourage good behavior. James Furzer

JF: If you take the Camden bench as a tickbox exercise, it meets its design brief. I'm not saying that the Camden bench is evil ... because it is successful in what it does. (The client wanted them) to use design to force out antisocial behavior. But then antisocial behavior is actually people congregating. We are designing people out of space.

I feel we need to design spaces that encourage good behavior -- and turn antisocial behavior into welcoming behavior. Ultimately, architecture isn't the cure of homelessness. There's a much greater issue with governments, properties and land laws.

I'm glad you brought up the homeless. Don't they need places to rest?

DH: I find it difficult to think why anyone would want to sleep on a bench. It's no place for anyone to spend the night.

JF: A bench isn't somewhere to sleep. But if we're excluding (the homeless) from sleeping on benches, then we need to include them somewhere else. We need to start designing our cityscapes with some sort of inclusive, secure areas. We just need somewhere they can get the security -- somewhere they can take themselves away from the prying eyes of the public. That must be so condemning, being stared at all day.

Architecture isn't going to solve the issue, but it can provide some sort of temporary solution just to give them some sense of well-being.

An armrest on a bench in Toronto, Canada. Credit: Cara Chellew

Finally, can the public enjoy urban spaces with hostile architecture?

JF: Yes, because (the general public) would have a different requirement from that space. Maybe you need somewhere to sit for five minutes, but you don't generally need a place to stay for a long period of time.

DH: Yes, I think the public can enjoy public spaces if they are defensive. It depends on how well it's done. If you have no shop doorways, or (places like that) where people can linger, people will still go through those areas and use them, as they would normally. They probably just wouldn't notice that there isn't antisocial behavior going on.