High art: thousands marvel at the interiors of Glasgow's School of Art, but what about the outside?

• Lucy Creamer, one of Britain's most accomplished female climbers, and Dr Jonathan Foyle scaled every side of Mackintosh's art school building for the programme

But tonight at 6:30, BBC2 will offer a unique – albeit at times, stomach-churning – view of the design masterpiece of Charles Rennie Mackintosh. Climbing Great Buildings will allow viewers to ascend the imposing exterior of the Glasgow institution and see anew the flourishes that made Mackintosh one of the most influential architects of the past century.

Dr Jonathan Foyle, an architectural expert and art historian, was part of a small crew to scale the iconic building, an experience which he says has given him a fresh appreciation of Mackintosh. "Climbing a building like the Glasgow School of Art makes you look at things very slowly," he told The Scotsman. "It's easy to go on a tour of a building and march around it in ten minutes flat, and get a cursory view, but you won't appreciate the detail of the architectural work. Filming the programme essentially allowed me to see inside Mackintosh's mind."

Aided by Lucy Creamer, one of Britain's most accomplished female climbers, Dr Foyle went up every side of the building in a painstaking exercise, navigating his way around the imposing masonry, delicate ironwork, and vast windows.

During the climbs, he studied the "living, evolving space", pointing out the innumerable ideas captured by Mackintosh, whether it be the designs celebrating the Scottish baronial castle and Celtic mysticism through to work evoking Japanese heraldry and early Art Deco elements. "Mackintosh is able to reconcile all kinds of influences," he says.

Considered a key European building, and one which is firmly part of the cultural and architectural fabric of Scotland's biggest city, it seems there could be no fresh discoveries about the Glasgow School of Art, the last complete building designed and built by Mackintosh.

Dr Foyle, however, was delighted to pick up on little details from his new vantage points. Above all, he believes they show the playfulness of the man behind the building.

He explained: "Mackintosh likes having fun, I could see him poking fun at traditional architecture. At one point in the climb for example, I noticed an arch where should have been a keystone to keep it in place. Except with Mackintosh, of course, there was no keystone.

• Glasgow's School of Art

"It's no wonder that most architects regard it as one of the most important buildings of the last century. In the same way that Turner is the painters' painter, Mackintosh is the architects' architect."

While his architectural knowledge is widely respected, Dr Foyle, 40, from Stamford in Lincolnshire, still expresses bemusement at being asked to present the BBC2 series. As someone with no climbing experience, Dr Foyle – the former buildings curator at Hampton Court Palace and Kew Palace – thought the programme makers had made a mistake when they first approached him.

"I was sitting in my office when I received a call from a producer letting me know that I was on the shortlist for a new series about climbing," he recalled. "At first, I thought it best to tell them they had the wrong number, as I had never climbed before in my life. But I heard some more about the series and it sounded interesting.

"I've been studying buildings for over 20 years, and until this series came about, I have had the same vantage point as anyone else. But climbing them gives you the very same perspective that the people who built it once had, and it offers a very real insight into some of the details."

The physical demands of the production shocked him. Armed with four climbing lessons and Ms Creamer's expertise, he said he was nonetheless exhausted at the end of filming for the 15-part series, which also saw to team ascend the likes of St Paul's Cathedral, Blenheim Palace, Clifton Suspension Bridge, and Durham Cathedral. Such were the challenges presented by the buildings, a 13-hour day was not uncommon.

He added: "It's all very well climbing at an indoor centre, with the brightly coloured nylon hand grips. But a building is a different challenge altogether. It's far more difficult for rope access, and some of the buildings – like the Glasgow School of Art – are very delicate.

As an architect, you're fearful of knocking masonry loose. The Mackintosh building was pretty tough. We climbed all over it, up the east and west sides, before heading up the north side with its beautiful ironwork great studio windows, and climbing up the library and the main gallery. We were roped between trusses, and you had to transfer between ropes all the time – it is the shortest climbs, I've found, which are the most difficult."

A dominant and cherished presence in the Garnethill area of the city for the past century, the Glasgow School of Art building was created by Mackintosh over a 12-year period that spanned the 19th and 20th centuries. The gap in the work, a financial necessity, allowed the architect to refine his vision, and much of the building's charm is in the fact that none of its faades are alike.

The earlier parts – the central and eastern half – were completed by 1899, following which Mackintosh made several visits to the school, reworking his original designs and overseeing a series of alterations and extensions, including the provision of a second floor of studios, and additional workshops accommodated into a sub-basement floor. By the time the funds were available to continue with the building, it was evident Mackintosh's vision had grown.

Internally, the most dramatic of the interiors was reserved for the library, with its decorated balcony and central cluster of electric lights. The room, says Dr Foyle, is one of his favourite in all of Europe.

Today, the Mackintosh building is not simply an architectural monument with a protected listed status. It is revered as a functional working space, where students work alongside an ever-increasing flow of tourists, with more than 20,000 people passing through its distinctive iron gate entrance every year on guided tours.

Little wonder, says Dr Foyle, that those fortunate enough to call the building home feel such an affinity to it. "The staff at the school all seem to own the building, they feel part of it. It's a special place."

Johnny Rodger, a lecturer at the school, agrees: "There's a contrast between the robust and powerful and the sensitive and delicate. The library is a timber room suspended in a sandstone building. Is it a Glasgow tenement or a castle?"

Dr Foyle is the chief executive of World Monuments Fund Britain and a specialist on Channel 4's Time Team series, and having enjoyed views of the building that are ordinarily the preserve of Glasgow's seagulls, he was amazed at the myriad influences employed by Mackintosh – a man, he says, who had a "magpie brain": "It's a very innovative, modern building which fits in with the city of Glasgow, in terms of the architecture of the tenements and the industrial character of the place,"

"In the main gallery, we descended down into the stairwell, where countless people have been. But the position we had allowed us to see an iron sculpture telling the story of St Mungo. It's little touches like that which are special."

With filming for the series now over, Dr Foyle says he misses the thrill of climbing. There is, he concludes, no better way of appreciating "human effort and engineering ingenuity".

"When you're standing at the top of Durham Cathedral, you may not know the names of the people who helped build it more than 900 years ago. But to be in that vantage point is the next best thing. It's simply miraculous."