Today, 11 January, the Guantánamo Bay prison "celebrates" its 12th birthday.

In case anyone needs a refresher, $4.7bn has been spent running Guantánamo. Nearly 800 men have been imprisoned, many losing over a decade of their lives. Nine have died. The world will never look at America in the same way again.

Barack Obama, who promised to close Gitmo in 2008, transferred out 11 men since the summer. These are men long since proven innocent, men too obese to walk and too schizophrenic to make any sense. These transfers are the first signs that the US may close a prison that exists to hold enemy combatants in the war on terror – a war whose battlefield, opponents, scope, and ending have never been defined. But the prison built to "protect our freedom" after 9/11 has made us no safer. According to Major General Michael Lehnert, Guantánamo's first commander, many of the detainees should never have ended up there at all.

I've seen Guantánamo's absurdities first hand. In the summer of 2012, I became the fourth artist to visit the facility. I saw the razor wire-ringed cellblocks in which we keep the 155 remaining detainees. According to a 2005 report by Seton Hall University (pdf), the vast majority of these prisoners were captured by Afghan and Pakistani forces, then sold to us for bounties. Of these, Guantánamo's chief prosecutor General Mark Martins told me, only 20 were even chargeable with crimes.

Medics with Shakespearean psuedonyms showed me the chair where, at one time, 45 hunger strikers were strapped down and force-fed twice a day. They were refusing food to protest their indefinite detention. The US military had taken away their lives as they knew them. We would keep them alive by force.

Detainees are allowed to speak to their families, via Skype, only four times a year. The prison library bans many books, including Aleksandr Solzhenitsyn's Gulag Archipelago about Soviet forced labor. The librarian told me it might sow dissent. Instead, they offer handbooks on reducing stress. Military police showed me the room where hunger strikers were shackled, alone, to watch TV. A guard snickered about how the prisoners liked the show Top Model.

I visited Guantánamo twice, but I only saw detainees once, for seven minutes, through a one-way mirror. They were skinny, middle aged men, joking and praying. Detainees are forbidden to speak with the press. According to Rear Admiral Richard Butler, who is responsible for prison operations, this is to avoid "making a spectacle" of them, which is forbidden by the Geneva Conventions. But Brandon Neely, who served as a guard in 2002, told me that soldiers were instructed that the Geneva Conventions were not in effect.

Captain Robert Durand, a Guantanamo spokesman, assured me that detainees now attend interrogations in return for Happy Meals. In the past 12 years, all the information we have suggests that every detainee has been tortured. A 2002 memo by military lawyer Diane Beaver approved waterboarding, beatings, extreme temperatures, and making a detainee believe his family was in danger of death. Mr Neely told me he and fellow guards beat detainees. At the start of every shift superiors told them "[the detainees] would kill you and your families in a heartbeat".

In Guantánamo's courtroom, I drew Khalid Sheikh Mohammed and his 9/11 co-conspirators. He is being tried in a military commission system that has produced eight convictions out of the nearly 800 men who have been detained on the island. Though it is over a decade since 9/11, Mr Mohammed's trial has not yet started. Lawyers are still hashing out the new legal system – half military, half civilian – that President George Bush created for Guantánamo. For a week, they fought over co-conspirator Mr Bin Attesh's stomach problems.

Press watched the hearings through layers of bulletproof glass. Soldiers confiscated my opera glasses (brought to better see Mr Mohammed's face) as "prohibited ocular amplification". An official censor put stickers on my drawings before they were allowed to leave the room.

With several exceptions, Guantánamo's detainees are not criminals serving a sentence. They are enemy combatants, held until the end of the war on terror. But terror is not a nation – it's a concept. Colonel Morris Davis, who served as Guantánamo's chief prosecutor from 2006 to 2007, told me, "We never really had a discussion about when the conflict was going to end."

Meanwhile, we keep 155 men in cages, at the cost of $1.7m per man, per year. When they try to starve themselves in protest, we keep them alive with tubes shoved into their stomachs. Guantánamo Bay's official slogan is "Honor Bound to Defend Freedom". Like so much of Guantánamo, it is easy to mock. But freedom, like terror, is a slippery word. What is its meaning amidst Gitmo's cameras and razor wire – where captives, cleared to leave the prison for years, are only flown home in shackles and hoods?

Clifford Sloan, Obama's newly appointed envoy to transfer prisoners out of Guantánamo, recently told PBS that he was sure the prison would be closed in the foreseeable future. I suppose the 11 men that Obama released in recent months given some hope that this may come true.

But even if we close the prison, we must make sure we do not build new Guantánamos. America must never again start a war with no defined enemy. We must reject indefinite detention and offshore prisons. We must no longer use our fear of terror to inflict terror on the world.

America must no longer write "Honor Bound to Defend Freedom" on the walls of its most notorious prison. Instead, we must mean it.

• This article was amended on 12 January 2014 to correct the name of the former prison guard the author spoke with. He is Brandon Neely.