Decadence has its attractions, obviously. You would be inhuman not to be impressed by some of these spaces, though the debating chamber feels as though it is tilting backwards - an illusion caused by the oval spiral ramp (in fact a shallow staircase) above it. This winds upwards to the viewing gallery and function room on top. It is the building’s most effective one-liner, and it certainly keeps you fit. By the time you have walked from the bottom to the top and back again, you have covered exactly a kilometer. Packing all that walking distance into such a small and compact building is another of Foster’s curiously irrelevant achievements here. It could be done, so it was.

Foster’s partner in charge of this project, Ken Shuttleworth, remarks that the whole of City Hall would fit inside the debating chamber of the Reichstag in Berlin, one of Foster’s most significant projects of recent years. Indeed there is much of the Reichstag condensed into this £40m building. The Reichstag has a circular debating chamber, offices for MPs and staff, and a glass dome on top with a spiral viewing ramp for visitors. In London, these ingredients have all been tipped into the equivalent of the dome alone. Which is then pulled into a funny shape. A shape generated by computer to be the most efficient, climate-wise. As so often with Foster, good or bad, this is a built diagram.

It is commendably energy-efficient not only because of its shape and north-facing orientation but also because it uses London’s abundant ground water to cool it, flush lavatories etc, and recycles heat in winter, rather than depending on wasteful air-conditioning. Every big building in London should do this. Unfortunately City Hall is a pimple of a building sitting in one corner of a vast, £750m commercial office and hotel development, also designed by the Foster office. Another similarly huge new Foster office scheme faces it diagonally across the river. These massive buildings do not embody the low-energy principles of the little City Hall, because commercial developers demand more conventional, less up-front-costly cooling systems. If even the mighty Foster office cannot change this short-term thinking in the commercial sector, we’re not going to get very far with saving the planet.

Everything in City Hall comes back to the shape. The cellular offices are wedge-shaped, to fit in with the curve. Functionally speaking, this is not an ideal shape and leads to some clunky internal details, as does the curving of the spiral ramp - which is anything but elegant, close up - and the very clumsy wrapping of the strips of glazing around the outside. The exterior appearance of the building, as nearly always, belies the architect’s computer-derived images of transparency. This is because architects like to believe that glass is wholly see-through. In reality - especially when used on the slant like this - it is like a car windscreen seen from outside. It reflects, it turns solid. Moreover, this is cheap greenish glass. Advanced, optically clear glass is available and has been used with effect by Richard Rogers in his recent City buildings, but that was ruled out here as too expensive. Even had they used it, the weird shape would stop you getting much of a view in. The upshot of all this is that you do not get a transparent dome-like object. You get a grey blob.