In a narrow Cambridge street opposite Sainsbury's is a wall of elderly cement and behind that is a secrecy of gardens, cloisters and Elizabethan quads. Presiding over this hidden world is Colin Maxted, who considers himself the luckiest man in East Anglia. "I recently came off night duty," he says, "and I thought, who would believe that, while they were sleeping, I'd discovered a dwarf in a wardrobe."

Life as a Cambridge college porter is an invigorating melange of the mad and the mundane. Keys and fire hydrants are the principal ingredients in Maxted's role of steering Sidney Sussex College through an ordinary week. The walls of the cramped porter's lodge, last renovated in 1948, are fluorescent with keys, 2,500 of them, each with multiple codes and a coloured plastic tag. There is no escaping these keys. Even the tiny back bedroom, where the night porters berth during quiet moments, is stressfully hung with the spares of the spares, as well as with the black college gowns that fellows and students like to store there for reasons no one can remember.

Maxted likes talking about his key collection. The college holds a licence from the manufacturer for a unique cut of key that no high street key cutter can replicate. Only Maxted possesses the machine that can conjure new ones when uninitiated colleagues replace sets undiscoverably on the wrong hook. But the licence is nearing the end of its 20-year run and he is bracing himself to replace all 2,500 keys and the locks that go with them.

When he is not thinking about keys, he is marshalling fire hydrants and smoke alarms and all the myriad technical precautions against combustion. "I'm in charge of every fire extinguisher in the college," he says proudly, "and the safety and security of all 19 of our off-site hostels. It's a job and six-eighths on its own."

Banked on a wall beside the keys are CCTV screens displaying 32 different corners of the college and below that are ledgers cataloguing health-threatening mishaps, actual and potential. There's a lot of potential for accidents in this three-acre warren of slippery cobbles, curling staircases and medieval cellars where over 500 sometimes inebriated people study in term-time.

Maxted's job is to anticipate and prevent, and he's just launched a life-saving device for college porters – a rucksack. "To unfurl the college flag we have to lug it up the narrow staircase behind the porter's bedroom, clamber out of a garret window, teeter along the castellations and climb a metal ladder on to the roof, on which the flag pole stands 98ft high," he says. "Once the rope broke and I sat with a needle on the roof top stitching a new length to the old one. It was December and it was not pleasant." Henceforth a specially designated rucksack will carry the flag and free up the hands of clambering porters for self-preservation.

Maxted, 55, has always wanted to be a college porter. Born and bred in Cambridge, his appetite was whetted when a retired friend relaunched himself into the role and relayed his adventures so enticingly that Maxted was hooked. "I was 28 and I rang up a college and asked if they'd give me a job, but they told me to come back when I was older."

And so he became a master butcher with a large company and rose through the ranks to senior management until, overnight, he became a single parent of three and he needed a job that would release him for the school run every day. His old dream resurfaced and, nine years ago, he was admitted into the inner sanctum of Sidney Sussex. "Everyone thought I must have read Porterhouse Blue," he says, "but I hadn't. I just felt it would give me sense of belonging – to a college and to Cambridge."

His passion for keys, I reflect, is symbolic. Most of his Cambridge life has been spent outside the walls of the university that dominates the small city; now he has found a way in to a rarified realm.

He says the traditional divide between "town" and "gown" has never disturbed him, but has his embrace of both changed his perception of his home? "I feel part of a family again," he replies. "I'm on first-name terms with everyone and that's not the same in every college, I can tell you."

His status as town rather than gown can be helpful as Oxbridge struggles to alter the public-school face of its intake. He makes it his duty to ensure that "outsiders" fit in. "I saw a mother and a son crying on the college lawn," he says. "She told me she was a single mother from a Birmingham council estate and she didn't feel they belonged. I told her that her son has earned his place here and has the same rights as everybody else."

I am disappointed that there are no bowler hats as worn by his intimidating colleagues at the grander colleges. It's formidable how much authority is borne by a bowler. But Maxted's naked head doesn't worry him, "although I do have a bit of a thing about homburgs!" His authority is sourced naturally from within, although he prefers avuncularity. That's where the weight of years is an advantage. One of his nine-strong team is a retired bank manager, another a metals expert and Maxted reckons a previous career and preferably parenthood are essential for a role that requires a strange mix of subservience and authority. One hour he is doffing his notional bowler to the college fellows and calling them "sir"; the next he's removing from the premises a fellow from another college who has arrived illicitly with a team of intoxicated rowers.

"I have an advisory role with the students. I'm not their dad, but if they don't act grown up they get admonished or, as we say, 'put before the dean'. This morning a crowd of ex-students came through in a state of undress as pirates so I know there's something afoot, but I like to think that I'm young-minded and I turn my eye from a lot."

Everyone has to pass through the porter's small realm on entering the college – or they will do when Maxted has his way and a detour through an archway is closed off – and that's how he likes it, even if continual interruptions about lost keys and replacement light bulbs distract him from his ledgers. "If you see the students regularly you know if there's a problem," he says. "And if there's a problem, I offer them tea and a chat. They're often dead nervous about being away from home for the first time – some of them have extra difficulties like autism – and we're part of their journey."

It's at night that the dramas happen. "Almost always involving water," he says. "A leak or a flood or the time I found a crocodile in one of the bathrooms." He has had to wake a student to tell him his father was dying and search the college for a missing girl who had taken an overdose. At night, too, are the social events when Maxted transforms into police guard and bouncer, manning the doors to the rococo dining hall and ejecting the inebriated. "Some young people have just left home and never drunk port before," he says, "and some of the older ones think it's funny to get the first years drunk." The obvious solution is matrimony. "Second and third years get 'married' in a ceremony in the cloisters and they are given a first year to 'adopt'. It's a mentoring system, Cambridge-style."

By day, the routines are less thrilling. Maxted paces the college precincts with a bunch of two dozen keys checking fire exit signs and emergency phones. He labels pigeon holes and directs errant tourists. His pride in the ancient buildings and his love of his work bursts from him with eloquent frequency. He leads guided tours for free and puts himself voluntarily on call 24 hours a day. His greatest dread is retirement and he hopes to work unsalaried beyond it.

I am privately amazed. The keys and the screens and the many logs and ledgers provoke in me an urgent desire to flee. But then in bursts a riveting cleaner with purple hair and turquoise toe nails who recalls the day she found a human skeleton in a student's bed. She too dreads impending retirement. "This is my family," she says. And I realise that the sense of purpose and achievement that pervades an elite academic community pulses down the ranks and can turn a floorcloth into a tool to advance human progress.

"I think of myself as like the cleaner for Nasa who, when asked by Ronald Reagan what she did, replied, 'I help put men on the moon'," says Maxted. "Even though what I'm doing is tiny, it help keeps the whole place ticking over smoothly."

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