If you go down to a campus today...

Durham Castle is highly unusual in being both a student residence and part of a Unesco World Heritage Site, but for anyone staying the night, it can feel like a rather spooky film set. The 11th-century chapel, vast Great Hall, Tudor kitchen, gatehouse and keep are undeniably impressive. Yet it is a strange experience to make one's way to bed up the dark, creaky, wooden stairs, past suits of armour in ill-lit corridors. It is particularly disconcerting to be put up in the bishop's bedroom because, although it hasn't been used for centuries, the current incumbent could, in theory, turn up in the middle of the night.

Not every university in Britain can boast its own castle, or even a classy chunk of crenellation, but quite a few can. Airthrey Castle was built in the mid 18th century and then greatly extended in the late 19th, with a panelled oak hall, alabaster plaques and carved eastern doors. It is now used by the University of Stirling's School of Law.

Horace Walpole, son of Britain's first prime minister, turned a cottage at what is now St Mary's University College into his personal "little Gothic castle" and incorporated it into his pioneering Gothic novel of 1764, The Castle of Otranto. The Founder's Building at the heart of the women's college that developed into Royal Holloway, University of London, has a dramatic roofline largely inspired by one of the Loire Valley chateaux.

Yet castles, specially built or acquired, are just the tip of the iceberg. British universities have many strange, striking and sometimes iconic buildings and other architectural projects to their names, from an egg-shaped lecture hall (Edinburgh Napier) to an "infinity bridge" (Durham) - specially designed so that, when illuminated at night, it links up with its reflection to create the symbol for infinity. Before the economic slowdown, the expansion of the sector witnessed a number of fabulous new libraries, laboratories and campuses, many of them celebrated in the pages of Times Higher Education.

Just as interesting is the way that, through the accidents of history, our universities have inherited a vast variety of buildings. It is unlikely that anyone planning one from scratch would choose to incorporate an anchorite's cell, the chapel of a Jesuit naval college, a collapsing railway tunnel, a defunct observatory, a fire station, octagonal stables, a royalist fort or a Victorian bathhouse. Yet all have been more or less successfully adapted to the needs of 21st-century higher education.

Planning restrictions mean that some very impractical edifices have to be preserved. At the extreme end of the scale are the rich men's follies whose whole point was to be charmingly pointless but which often attract the attention of English Heritage. The University of Bristol's Goldney Hall, for example, was originally built in the 18th century as a country home for a Quaker merchant family, who made much of their fortune from privateering raids on Spanish treasure ships. It is set in 10 acres of formal and wooded gardens laid out by Thomas Goldney III and became a hall of residence for 19 female students in 1956.

Along with an orangery, orchards and "old-world garden", its follies include a bastion, rotunda, Gothic tower, ornamental canal and statue of Hercules. Most spectacular of all is the grade I-listed grotto, where the walls are encrusted with a lavish display of minerals, shells, corals, rocks and fossils. It is probably hard to find an educational use for it.

Other dramatically landscaped gardens owned by British universities include those attached to Roehampton's Froebel College, complete with lily pond, ornamental bridge and mausoleum stretching down to an artificial lake, and the terraced Italianate gardens surrounding Exeter's Reed Hall (although the palm house has been relocated to a nearby hotel).

Tight links between universities, medical schools and hospitals have also left their own complex heritage. The former operating theatre of the Royal Northern Infirmary is now the vice-chancellor's office for the University of the Highlands and Islands, while a ghost is said to haunt the part of Northumbria University's Sutherland Building that once housed the morgue. More or less "secret" tunnels for transporting patients and cadavers between buildings can be found at the universities of London and Edinburgh. The tunnel at Edinburgh was used by the celebrated "bodysnatcher" William Burke, who took to murdering people so that he could sell corpses to an anatomy lecturer keen to sharpen his students' dissection skills.

Given that alcohol and sport are so much part of the student experience, it is unsurprising that they also feature prominently in universities' property portfolios. The London School of Economics has acquired two local pubs over the past decade, serving staff, students, lawyers and others working in the nearby Inns of Court as well as passing trade. The George IV is owned and managed by LSE Catering, with the three upper floors turned into flats and rented out. Ye Olde White Horse is managed by tenants on a long lease, although LSE Catering set up a new wholesale business to supply the alcohol.

The University of Bath has a unique 125-metre push-start track, simulating the start of a full bob-run, where British Olympic bob skeleton and bobsleigh squads have honed their skills in the absence of any convenient mountains. Although it was opened in 2002, staff from the Department of Engineering and Applied Science have continued to make modifications so it remains state of the art.

Only this autumn, when Wrexham Football Club was hours away from being kicked out of the Football Conference, Glyndwr University stepped in to buy the 15,500-capacity Racecourse stadium - said to be the oldest international football ground in the world - and Colliers Park training facility. As well as saving a celebrated local team and source of communal pride, this should help to serve longer-term goals by laying the foundations for an enhanced sports science programme.

But what are the most unusual types of building or architectural feature owned by a UK university? It is worth exploring two particularly striking examples.

Many institutions record their war dead, although the North Wales Heroes' Memorial Arch at Bangor University lists on oak panels the names of more than 8,500 soldiers, sailors and airmen from the region who fell in the First World War.

Far more surprising is the Jewish cemetery incorporated within the campus of Queen Mary, University of London. This was opened in 1733 as the Nuevo Beth Chaim, after the nearby old cemetery - created in 1657, just a year after the Jews were readmitted to England by Oliver Cromwell - outgrew the available space. (The Mile End location was then a mile beyond the city limits, as religious law requires.) It remained in use until the early 20th century as the main burial ground for the vast majority of Sephardi Jews (descended from those expelled from Spain in 1492) who were not grand enough to afford mansions and private mausolea.

By the 1960s, what is now Queen Mary was looking to expand eastwards. This led to an extended process of negotiation with the Spanish and Portuguese Jews Congregation, an Act of Parliament and eventually, in 1974, agreement on a long-term lease. Bodies in the older graves were disinterred and buried elsewhere, while the college, as it was then, took responsibility for maintaining the later section as an open space. The new arts block then grew up around it.

Caron Lipman, a geographer who recently completed a PhD at Queen Mary and has since been studying the cemetery, describes it as a "fantastic historical resource" where she guesses that between 600 and 700 people are still buried. It is fenced in and largely neglected, although relatives come occasionally to place pebbles on the graves. Now, however, Philip Ogden, professor of geography, has spearheaded plans to take better care of it by planting trees in raised pots, creating a seated area and incorporating a pathway with explanatory plaques. Lipman has also produced a forthcoming book on the subject, including case histories of some of the individuals buried there.

Stranger still is the remarkable combination of airport and sewage treatment works owned by Cranfield University in Bedfordshire. The former came first, in 1936, in response to increasing tensions in Europe and was used as an operational training unit during the Second World War. The national postgraduate College of Aeronautics was built round it in 1946 and expansion into other technological fields eventually led to the creation of the university in 1970.

The airport is now wholly owned by Cranfield as a separate "school" within the university and operates on a commercial basis under an "ordinary" licence, which means that holidaymakers' flights don't stop there and there is no need for terminals and security arrangements. The main users are flying schools and clubs and those offering private pilot training.

Director Jason Ivey estimates that they see 2,500 movements of business and executive aircraft per year. The Facility for Airborne Atmospheric Measurements, a collaboration between the Met Office and the Natural Environment Research Council, owns a British Aerospace 146 used for atmospheric research, which was the only British aircraft flying in the aftermath of the Icelandic ash cloud. There is also a "flying laboratory" in which aviation and aeronautics students can take to the air and carry out research, although any such educational uses of the airport form a very limited part of its activities.

Like the university itself, the sewage treatment works were an offshoot of the airport, although they needed to be upgraded as the college evolved into today's Cranfield. Water and waste requirements for a community of more than 4,500 staff and students are now served through an on-site plant rather than by municipal resources.

This is said to have both environmental advantages and educational value, since - according to a spokesperson from the university - it means that the sewage plant can "serve as a teaching facility for research work for students of the university. Research is therefore conducted on real waste water - rather than synthetic waste water - which many other research groups have to use." Although this is no doubt a selling point, it probably doesn't deserve too prominent a place in the corporate brochure.