Okay, the fun is over. Now comes the hard part.

Authorities in New York yesterday announced the winner of history's most visible architecture competition. A moving, innovative design by Studio Daniel Libeskind will be the blueprint for the redevelopment of the charged territory where the World Trade Center once stood.

This was a most excellent choice. The Libeskind plan has the potential to transform the 16-acre site in Lower Manhattan into something truly special.

More profoundly than any of the other eight proposals in the high-stakes competition does it respond to the emotional content of the place. The very placement and shape of the angular skyscrapers and public spaces translates this energy into architectural and urbanistic terms.

Nor is the complex design just about commemoration. The Libeskind plan contains the seeds of a remarkable rebirth of street life and public spaces where once there was a vast, unfriendly plaza. And with a slender, 1,776-foot-high structure crowning a cluster of beveled office towers, it promises a vivid, rejuvenated skyline.

Better than any of the other entries, this design promises to balance the competing claims of past, present and future. It allows for meaningful remembrance, yet remains open to the enticements of the moment, of the living city all around.

Now, managing to get something built with anywhere near this level of inspiration, that will be hard.

For one thing, like most competition entries, this one is more a sketch of the future than a final plan. The details are yet to come and, as Ludwig Mies van der Rohe famously (and rightly) said, "God is in the details."

It will matter a lot just how finely these buildings and landscapes are made. It always matters a lot, of course, but maybe even more so with designs as unorthodox as Libeskind's. When real estate developers approach with formulaic concerns -- square an angle here, use a cheap material there -- Daniel Libeskind is going to need some protection.

This presumes, of course, that a plan along these lines has more than a snowball's chance in the Sahara of actually being built.

The Port Authority of New York and New Jersey and the Lower Manhattan Development Corp., the agencies that together produced the requirements given to all competitors, put an enormous amount of office space high on the list. The primary leaseholder of the World Trade Center towers is insisting on a huge quantity of office space as well, to replace what once was there.

It may be that, eventually, the future of this site in the financial district lies almost entirely with high-priced, high-density office occupancy. But maybe not. The office vacancy rate was high before the attacks on Sept. 11, 2001, and it has gone higher since then.

Are there not some more plausible, as well as more attractive, futures? Before 9/11, residential occupancy was growing a lot faster in Lower Manhattan than anywhere else in the city. New York Mayor Michael Bloomberg urged his planning agency to envision a future for the whole of Lower Manhattan, and it came back recommending a more vital, mixed-use place.

In addition to a fixation on office space, the two agencies would appear to embrace the idea that folks love to descend to shop, because underground is the only place to put the bulk of the whopping million-plus square feet of retail space they would like to see. The retail leaseholder, a developer of large shopping malls, loves the notion, too.

But it's not that great an idea. Some underground shopping is needed, for sure, to service and enliven the subsurface transit system. But, just as did the retail center underneath the trade center plaza, an underground mall of this scale would siphon life from the streets.

Recently the Port Authority has been pushing a vast underground parking lot for buses. Again, some buses, sure, but hundreds every day? The old rule about freeways and traffic -- if you build the roads, the cars will come, and come and come -- applies with special force to a monster underground garage for pollution-belching, space-consuming tourist buses. Early in the 21st century, isn't there a better way?

One of the strengths of the Libeskind scheme is its flexibility. That is to say, despite its complexity and subtlety, it can be adapted to account for changed circumstances. This can work both ways -- the plan can be altered for the better or the worse.

On the one hand, with a little bit of work those beautiful, angular buildings could be built for residential rather than office use.

Or that soaring tower, planned as an office building with public uses at the upper levels -- a sort of international botanical garden in the sky -- could be refined. I like the symbolism of the top, but question the practicality and desirability of what the architect refers to as the "gardens of the world."

On the other hand, to fit that bus garage (and the existing underground trains) into the plan, Libeskind had to make significant alterations to the memorial space he had envisioned. In the initial proposal it was burrowed to bedrock, 70 feet deep. In the revised design selected yesterday, it goes down 30 feet.

This particular change may not prove harmful to the design. Probably, enough of Libeskind's original concept remains to make a powerful setting for the memorial. (The design for the memorial itself is to be chosen from an international competition later this year.)

But it's a cautionary tale. Pressures such as these are commonplace. The essentials of Libeskind's design need protecting right from the get-go. In particular, these include its convincing memorial concepts and spaces; its splendid sequence of public places above, below and on the ground; and its refreshing architectural verve.

Basically, the whole process of choosing a design, though sloppy at times, has been so far a pleasant surprise. The outpouring of public sentiment last summer, after the release of six rather humdrum urban design schemes for the site, forced the issue of quality architecture to the front. Then, the invited competition, involving some of the most innovative architectural talents in the world, turned out not to be a mere beauty contest.

Yes, the images were striking, and yes, they made front pages and television newscasts around the world. People tuned in. The official Web site, www.RenewNYC.com, has received some 8 million hits since the designs went on view in December.

But underneath those handsome, computer-generated images, thank goodness, there was a lot of serious thought. Imaginative architects were able to prove what they -- and architecture -- could do in response to a shocking event and a complex new reality. Libeskind's design is the worthy outcome. It is a great beginning.

Of the nine proposals considered, the Libeskind design for the World Trade Center site best provides both memorial and utility. But its acceptance is just a first, and relatively easy, step.Studio Daniel Libeskind's design combines beauty with great commercial utility and flexibility of use.