The ornately carved oak door that can be seen at Brunel University's Runnymede campus was once very familiar to the Victorian engineer who gave his name to the institution. Indeed, Isambard Kingdom Brunel is reputed to have carved it himself, though few believe that the man who pioneered suspension bridges, ocean steamships and railways could have found the time to undertake such an intricate job. It is more likely to be the original front door of the home in Duke Street in Westminster, where Brunel lived from 1836 until his death in 1859. The door was bought in the 1960s for ?10 by the university's first vice-chancellor, and it wound up mouldering in a wood store until it was rescued, restored and put on display on the first floor of the President Hall at Runnymede.

One of the oldest doorways in academe is at University College, Durham (pictured, first from left). The late-Norman decorated archway dates from about 1150 and was once the front entrance to Durham Castle, originally reached by a wooden staircase that could be burned in case of a Scottish attack. Much of the elaborate decoration survived down the centuries because it was buried under plaster by a disapproving Prince-Bishop. The door was then incorporated into an enclosed gallery in 1540. In 1837, the castle was transferred to the new Durham University and became a student residence. The full beauty of the archway re-emerged only a couple of decades ago during restoration work.

Standing alone in the back field behind the main entrance to Goldsmiths College, University of London, is a strangely familiar portal (second from left). The shiny black surface with its ornate knocker is reminiscent of the famous door to 10 Downing Street. Yet the steel slab is heavy and immovable, a barrier not a gateway. Norwegian artist Svein M?xvold who constructed the door for the college's fine art degree show set out to turn a symbol of democracy into "an arrogant obstacle for common people - you can knock but they never let you in". The 250kg sculpture is set to remain in the field, at least for the next few months.

Above the door at the back of Bristol University's physics department are carved two classic scientific experiments (third from left). On one side, sunlight is being split into the spectrum by a prism; on the other, radioactive particles are streaking across the stone. The door was designed by the architect Sir George Oatley for a grand laboratory complex commissioned and funded in 1913 by the university's pro-chancellor, Henry Herbert Wills, whose fortune came from cigarette manufacturing. Arthur Tyndall, Bristol's professor of physics, wanted to make a statement over the main door and Oatley came up with the idea of commemorating the first great English physics experiment - Sir Isaac Newton's splitting of light, and the latest - C. T. R. Wilson's tracking of alpha and beta particles in a cloud chamber. In the 1960s, when the department was expanded, it was reconfigured to front onto the nearby road and so Oatley's creation became the back door.

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The trapdoor in the tapestry room in Firth Court at Sheffield University was devised to thwart protesters. It was secretly installed during the era of student occupations in the 1970s and '80s after one sit-in resulted in university council members being in effect penned inside the room for several hours. The trapdoor was a means by which members of the council could escape, opening onto a spiral staircase leading to a basement once used to store books. Academics might then exit the building via a rather nondescript door. In the event, it was never needed.

Hanging 3m off the ground on the side of the modern concrete exterior of Glasgow University's Hunterian Art Gallery is a white Victorian front door (fourth from left) This was once perfectly functional, used by Charles Rennie Mackintosh, the architect, designer and artist, at his home in Florentine Terrace. The whole building was bought by the university in 1946, and its interior was stripped out before the terrace was demolished in 1963. The Mackintosh House was later rebuilt in the gallery, but due to the steepness of Hillhead Street, its front door became redundant - access is now possible from the side.

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Another unusable door can be found in the first-floor Old Library at Trinity Hall, Cambridge (fifth from left). It leads nowhere today. But soon after the library was built in 1590, it opened onto a bridge that the college's master had built across the courtyard from his lodge to save him the effort of walking up and down two flights of stairs to get a book. The link was taken down in the 18th century and the door was bricked up. But in the early part of the 20th century it was uncovered and can now be seen, complete with Tudor ironwork, from the outside of the Old Library, opening into thin air.

The great hangar doors at Cranfield University once opened to a host of Royal Air Force fighters and bombers (sixth from left). During the Second World War, the four C-type aircraft sheds were used to store, repair and refit Spitfires, Beaufighters and Typhoons, among others. Now the buildings are part of the university, housing aircraft, wind tunnels, engineering equipment and academic offices. The doors are 42m wide and 10m high, divided into six 14-tonne sections and are opened along steel rails by hand. During the war, they were even heavier, being filled with sand to protect against bomb damage.