The Barbican is, depending on when you start counting, 20, 40, or 50 years old. That’s 50 years since the first designs for what was a 35-acre bombsite were published, 40 years since construction began on a revised design, and 20 years since the whole thing was finished with the opening of the Barbican Arts Centre, when the Queen described it as “one of the wonders of the modern world”. By then, of course, it was monumentally unfashionable, monumental being the key word. Architecture had long since moved into its high-tech phase: lightness and transparency, steel and glass, were prized by then, not heavyweight concrete and brick. Now, of course, it is once more appreciated.

The architects of the Barbican, a firm called Chamberlin, Powell and Bon, did some other good work, especially for universities, but this project took the best of their lives. At the Barbican’s own anniversary exhibition, “This Was Tomorrow”, you are struck at once by the certainty that pervaded everything: not least in the quality of the original drawings. Immensely detailed and beautifully executed, these pre-computer exercises in spatial design seem like the work of another race. How did they get their heads round it all? How come they were allowed to design everything down to the doorknobs and washbasins?

It was a more trusting time. The Barbican was built pretty much exactly the way its architects wanted it, all influences intact: complete with serrated triangular-plan skyscrapers - inspired by Frank Lloyd Wright - and concrete barrel-vaulted flats, in homage to Le Corbusier, and lots - though not enough - good landscaping. It was and is a manifesto statement against suburban sprawl. It is concrete - but expensive, bespoke, hand-tooled concrete. Peter Chamberlin, Geoffry Powell and Christof Bon (all now dead) wanted cities to be like cities and the country to be like the country. They didn’t fancy mixing the two.