The towers were reviled by the critics either as being supremely dull, or as being too fussy. For critic Ada Louise Huxtable, it was “the world’s daintiest architecture for the world’s biggest buildings”. She did not mean the overall form of the towers, but Yamasaki’s busy latticework exo-skeletons. The buildings were framed tubes, with the load carried by the exterior. Each was structurally like a giant perforated box-girder standing vertically. The interiors were column-free. The central core played little role structurally, serving mainly to reduce the floorspans and provide lifts and services. Windows, inserted in this forest of external columns, were only 22 inches wide for most of the height of the towers. Critics protested that this removed much of the point of such towers - the view. Yamasaki replied that office workers were not meant to stare out of the window all day, and that anyway the narrow fenestration provided a feeling of security.

Because of the tall pointed arches at the bases of the towers - where Yamasaki drew every three columns down into one - Huxtable described the complex as “the ultimate Disneyland fairytale blockbuster. It is General Motors Gothic.” There was much more in this vein. The towers were acclaimed for their technology, but won not a single architecture award.

Yet Yamasaki had invented the third-generation skyscraper. His was the concept of the “sky lobby” - where instead of having elevators rising the full height of the building, a railway-like system of express and local elevators, with interchanges, was devised. The large express elevators rose to skylobbies on the 44th and 78th floors. Local ones served individual floors. This system was devised to speed the flow of people - the centre could accommodate 50,000 people in both towers, with a further maximum 80,000 people movements per day - but also meant that the towers were very space-efficient. Conventional elevator arrays in skyscrapers meant huge areas of space were swallowed up on the lower floors. Yamasaki’s big idea was to make each of his towers effectively three separate towers stacked on top of each other. The lift and service cores were much reduced, and net lettable area much increased. The World Trade Centre was 75 per cent space-efficient at a time when conventional towers achieved around 52 per cent.

This may seem mere developer-speak, but the design saved lives when the towers were destroyed on Tuesday September 11, 2001 by terrorist attack. During the all too brief period when the towers had each been hit high up by hijacked airliners, but were still standing, lifts continued to operate to the lower levels, allowing at least some people to evacuate.

Yamasaki and his engineers had designed the towers to withstand 150 mph hurricanes. Although plane strikes were not considered a likely threat, it was thought that the towers could handle such an event, and notices in the public observation gallery in the south tower said as much. This was true up to a point. The enormous lateral strength of the towers proved able to absorb the impact of the airliners, even though they sliced through the external structure. Reports from survivors record how the towers lurched and then stabilized.

But no steel-framed tower could withstand the intense heat generated by the firebomb effect of the planes’ exploding fuel tanks. Their almost-full fuel loads made them cruelly effective missiles. The steel columns were fireproofed with sealed-in asbestos (the effects of this asbestos in the dust-storm provoked by the towers’ collapse may have health implications for years to come), but this far exceeded any conventional fire. The columns would have turned to rubber in the fire zones. The progressive collapse that followed in each tower then became inevitable, for they were designed to support gravitational and operational loads, not vertical impact. The effect of the top sections of each tower falling onto the lower sections, as the columns in the fire zones below gave way, was like giant vertical hammer blows. Moreover, the latticework system of linked columns depended upon the integrity of the whole. Once the whole thing began to unzip, it weakened very rapidly.