Oslo, Norway - As a prison guard for the past 38 years, Lasse Andresen has seen it all.

He's worked in five prisons in Norway - low and high security, rural and urban, old and new.

Along the way, one of his colleagues was raped and killed by two inmates. His two daughters are now prison officers themselves. Now, as he nears retirement age, he finds himself at Halden Prison, reputedly the most humane prison in the world.

Walking under birch trees on the prison's peaceful grounds, Andresen sums up what he has learned on the job.

"Most prisoners you can get to trust them," he said. "But some you can never trust - the ones who have something wrong in the head."

When it comes to trust, Halden clearly has opted to give its 250 inmates the benefit of the doubt.

This high-security prison - one housing murderers, rapists and paedophiles - has no bars on the cell windows.

Humane facilities

The Norwegian prison system as a whole rejects punishment, but Halden pushes the humane ethos to its limits, housing inmates in airy rooms with en-suite bathrooms in a comforting, bubble-like environment, with enough education and leisure to keep them busy - and happy.

Take the prison's recording studio, kitted out with big amps and Fender guitars.

One year since the massacre

An inmate named Knut leans back on a swivel chair.

"Knut is here all the time. He's writing 20 songs," said prison officer Rune Ulfing.

The pair play around at the mixing desk. Were Ulfing not wearing a uniform, they might easily come across as friends. The mood is light-hearted, just like the logo emblazoned on the drums and equipment - "Criminal Records".

"It had to be that," said Ulfing.

In a prison where half the guards are women, gender is no barrier to friendship.

Prison guard Tina Bratas regularly plays cards, backgammon and ping-pong with inmates.

But, beneath the soft exterior, the institution has a steel core.

"If I feel threatened, I know I have 30 guards in under a minute to help me. And, the inmates know it as well," she said.

In any case, "most of the threats are always over the [intercom]. They are not tough enough to do it face to face".

No handles

There is a sense of freedom at Halden, said Andresen. But, he pointed out, it is still a high-security prison with requisite restrictions.

Internet use is heavily regulated. Inmates are only allowed 20 minutes of calls out each week. Letters are opened and read before being handed over. Then there are the walls, just about visible through hectares of woodland, and the strangeness of walking into a college-style environment to find the doors have no handles on the inside.

"When you first arrive here, you're, like, where's the handle?" said Carsten Fredriksen, who arrived two-and-a-half years ago to serve a 16-year sentence.

He would not disclose his crime. He lives in an apartment-style unit, featuring a tastefully-furnished living room with a flat screen and Playstation.

You would never guess you were in a prison, until you turn to the clinical, white corridor leading to the cells.

The Halden lifestyle is structured to be as normal as possible.

Every morning, when his fellow inmates go out to school or work, Fredriksen stays behind to cook and clean, earning 60 NOK ($7.75) for each day's work.

He depicts a harmonious existence. "There's compromises to make, but in some strange way, we make it work," he said, adding that violence is extremely rare.

"It's really exceptional. I think the moral is just set that way and we influence each other."

Andresen stressed the importance of good relations with the guards."The inmates want to have it good with the officers," he said.

"At least here, you can get the help to change," said Fredriksen. "If you want to, if you're willing to, there's someone here to help you."

At Halden, help is never far from hand.

Prison psychologist Jan Berglund said new inmates often come to him with anxiety, a condition heightened when they become aware of the rehabilitation plan awaiting them.

"Putting life on pause" is not an option. "It's easier to deal with a bare room, not do anything, not feel anything, not say anything," he said.

His advice to prisoners suffering from anxiety was: "Start immediately to leave the prison inside, start preparing what will follow after you've served your time."

Moving on with life

It's a mindset that Robin Andreassen seems to have adopted.

The young man was arrested for murder and is currently in custody awaiting a sentence in Halden's most restrictive unit. If deemed appropriate, he will be moved to a more open area of the prison, where he said he hopes to start training to be a cook.

Right now, he is working in the kitchen, having been brought on board by fellow inmate Fred André Jacobsson.

"It's a trust thing," he said. "You prove you're trustworthy and you get the job."

While Halden houses some of society's worst criminals, its doors are not open to all.

Anders Behring Breivik, for example, will never be staying here.

While Breivik, imprisoned after the murder of 77 people in Oslo on the island of Utoya in July 2011, was judged sane in 2012, he looks set to remain in isolation at Ila prison, near Oslo, which houses the country's most dangerous criminals.

"Halden is not a prison for him," said prison director Are Hoidal.

Even Halden has its limits. But, in daring to push rehabilitation further than most countries, Norway has managed to claim the lowest re-offending rate in Europe - at just 20 percent.

In a country where life sentences do not exist, all prisoners will eventually end up going back into society. Halden wants to ensure they become good neighbours and taxpayers, said Hoidal.

"They call it the most humane prison in the world ... and we like that," he said.