The Metropol Parasol in Seville nearing completion last week: ‘From some angles it is a wonderful thing, daring, inventive and impressively consistent.’ Photograph: Sergio Caro for the Observer  Oh my God, it's an icon. How very last decade. Did the city of Seville not get the memo? Big, flashy buildings are out; hair shirts are in. Then again, building projects are slow things, especially when they have hugely ambitious and untried structural ideas. In 2004, when the Metropol Parasol project was launched, and Spain felt flusher than it does now, few were thinking it would open after the country was hit by one of the worst of the European Union's many financial crises. As it is, like the grandiose new City of Culture of Galicia complex in Santiago de Compostela, it looks like a late work of bubble baroque.  It is undeniably arresting. Last week, as workers scrambled to finish the building and remove all the scaffolding in time for the official opening on 27 March, citizens were gawping at and debating the 30 metre-high cloud/mushrooms/parasols/waffle that had appeared in their ancient city. Two passersby, men in late middle age, expressed the poles of the argument, one saying that it was out of place, the other that the city should move with the times.  The Metropol Parasol actually is a device for revitalising the Plaza de la Encarnación, for years used as a parking lot and seen as a dead spot between more popular tourist destinations in the city. The Parasol contains a market, shops, and a podium for concerts and events. In its basement are the ruins of a Roman district, with mosaics and enough bits of wall to get a sense of what the houses were like. On the roof there is a restaurant, a viewing gallery, and a winding, undulating walkway – a sort of pedestrian rollercoaster – from which to appreciate the views gained by rising slightly above the general roofline of the city.  These uses could be housed considerably more simply than they are by the new structure but, after the Roman ruins had wbeen discovered during excavations to build an underground car park, it was eventually decided to take the opportunity to make a statement. A competition was held, with an open brief and budget, for ideas for the site, followed by a more specific design competition. The winner was Jürgen Mayer H, a rising Berlin-based architect, now aged 45, who for reasons of effect has shifted his middle initial to the end of his name. This might be considered a tad pretentious, but no matter: he is a dynamic individual whose air of egg-headed seriousness can break into winning smiles. His striking designs have earned him the imprimatur of MoMA, which owns several of his drawings and models, and Calvin Klein, for whom he designed the setting of a fashion show.  Mayer started off as an artist, and still sees art and architecture as similar, being both about the interrelation of human bodies and space. He is interested in the world of the digital, and has created wallpaper based on the security patterns printed inside envelopes. His buildings have the complex shapes, with non-repeating elements, that computerised design and construction make possible: no two parts of the Parasol are identical.  The idea of the Parasol was to make shade, a valuable commodity in a city as hot as Seville, and so make the square more habitable. To avoid disturbing the Roman ruins, columns supporting the roof could only come down in a few places, requiring ambitious structure – designed with the help of the engineers Arup – to span the gaps between them. From these conditions came the trunk-like uprights, big enough to contain lifts and stairs, and a structural system using laminated timber and steel, held together with high-performing glue, tested to ensure it would withstand the highest imaginable temperatures in this spot. Among the project's boasts is that it is the world's biggest building to be held together by glue.  Mayer says the forms of his building were inspired by the vaults of Seville's expansive cathedral – he wanted to create a "cathedral without walls" that would be "democratic" – and also by the handsome trees already in the square. He could reasonably also point to other eruptions in the city's expressive, not-reticent architecture.  And so we have this – shhh! – this icon. It differs from others of its kind in that it is not a closed institution, locked at night, but a thing that can be used at all times. It is in the middle of a city, not on some industrial fringe straining for regeneration. It forms spaces under and around it, in conjunction with other buildings, rather than standing aloof and alone. It catches and holds views from side streets, and from inside the new structure looking out, and gives them intensity through the extraordinariness of the structure.  It is, seen from some angles, a wonderful thing, daring, inventive, determined and impressively consistent. It is also wonderful in its content, this stacking up of past, present and future, of ruins, market, performance space and sky deck. But it has a problem, which is that these two forms of wonderfulness do not connect, with each other or with their surroundings.  The market occupies an ordinary room much like a large lift lobby, not designed by Mayer, that is cut off from his extravagant architecture. The ruins are in a space designed by another architect, Felipe Palomino, who tries to achieve his effects subtly, through light and "sensation", and does not seem particularly appreciative of the boisterous stuff above. The things that should act as intermediaries between Mayer's thing and the life in and around it – balustrades, shop fascias, ceiling panels and paving – are ordinary going on shabby. Supposedly sinuous curves look scratchy and lumpy, materials meet thoughtlessly, and there is a lack of positive qualities in the detail.  At times the plaza feels like an inside-out mall or multiplex rather than a public space or a place of culture. This feeling probably reflects the fact that the project is run by the contractor Sacyr Vallerhermoso, which will also collect as much revenue as it can over a 40-year concession on the Parasol, in a version of the public-private partnerships beloved of modern British governments. Unless Sacyr is unusually saintly, it is likely that its desire for profit will sometimes conflict with the public purpose of the building. Access to the best bits, for example, may not be free, or cheap, and unlimited.  Mayer's design contributes to the disconnection. It puts too much faith in the power of look and shape, with the result that the appearance of fluidity masks – in fact, assists – a disjointed reality. The magic mushrooms demand both attention and energy: the complexity of the building contributed to its being many years in the making, and at one point it required a cash injection of €30m. The more everyday parts of the building are left looking eclipsed by the spectacles, and exhausted by the effort of achieving it. Which is a shame, as the Parasol is, almost, one of the smarter of the recent tide of iconic buildings.
Seville's flashy new showcase Metropol Parasol is held together by extra-strong glue – but not enough to make its many parts connect, writes Rowan Moore