The Arch, which has been slowly winched into place for a month now, gives a focus that was previously lacking to north-west London. There it stands, slightly tilted, a rainbow-shaped arch where, if the fancy takes you, you can imagine that all the colours have refracted back into pure white. Up close, you can appreciate the engineering might of the latticework steel structure, an open tube nearly 25 feet wide spanning 1,033 feet, rising to 436 feet. Partly it is like this so as to hold up a lot of the weight of the stadium and its part-retractable roof, but there are plenty of other ways to do that, as the far from elegant Millennium Stadium in Cardiff amply demonstrates.

A stadium is not a particularly difficult or radical thing to build - they are all reworkings of the Colosseum in Rome in various ways, they are all big, simple things designed to provide a spectacle and to get people in and out quickly, with various other things like restaurants and conference rooms tucked underneath the banks of seating. No, the new Wembley takes the form it does, with that great big arch, not because it is strictly necessary but because it could be done, and because it looks good. Seen from afar, the arch becomes a delicate affair - but not so delicate that, like the fragile-seeming observation wheel of the London Eye with which it holds a crosstown conversation, it dematerializes. No, it's there and you see it. Not another big roof, but an intriguing shape. People on the plane crane their necks and peer, question each other, consult guide books. What the heck is that, they ask? So clearly it is working. Such unique macro-scale buildings are ground-markers for the city. They tell you that this is London, not somewhere else.