The mid-1970s design of Dilworth Plaza by Vincent Kling was a pompous, overblown and hugely expensive affair.

The plaza was conceived as a way to free Philadelphia's magnificent City Hall from a noose of urban clutter, but Kling turned it into a vanity platform for admiring his adjacent high-rises - the Municipal Services Building, Centre Square and the late One Meridian Plaza. It wasn't for nothing that Philadelphia's civic heart was dubbed "The Klingdom."

We didn't know it then, but the plaza was Philadelphia's Big Dig. First proposed by Mayor Richardson Dilworth in the late '50s, construction didn't get under way until the late '60s. The project, which extended the concourse to City Hall, dragged on eight years and ran millions over budget.

The consensus today, after a mere 30 years of use, is that the indulgent granite composition is a colossal failure. Though the plaza succeeded in giving us a clear view of City Hall's richly sculpted facade, it never became a place where anyone wanted to linger. At best, it works as a crossroads.

A new design is now being developed by Paul Levy's Center City District. The plan, which is still a work in progress, not only aims to overhaul the plaza and transform it into a parklike fifth square for the public's enjoyment, but it would also dramatically upgrade the transit concourse below its surface.

Even in the early designs, it's clear that Levy's ambitions for the space could revolutionize the way we think about Philadelphia's dreary 9-to-5 municipal quarter. At the same time, it's also evident that the plan - a collaboration between no fewer than five Philadelphia firms - needs much more thought and refinement if the city hopes to avoid repeating old mistakes in a new form.

Kling's plaza had too much architecture and too little activity. Levy's may have too much activity and the wrong kind of architecture.

You can't fault Levy's reasons for targeting Dilworth Plaza, though, or his timing.

The plaza sits at Philadelphia's center of gravity, atop one of the busiest urban-transit hubs in the nation, and links West Market Street's offices with Center City's residential and entertainment districts. Thanks to the condo boom, residential towers now clamor at the gates of City Hall.

Once the expanded Convention Center opens in 2011, its main entrance will be one block north of the second-rate plaza. Meanwhile, SEPTA is getting ready to undertake a daunting reconstruction of its century-old City Hall subway station - whose ambience novelist Jonathan Franzen likens in The Corrections to a hospital emergency room.

SEPTA has spent years preparing for that project, petrified that it will open a Pandora's box of engineering challenges. But with Washington promising a windfall for transit, the agency can no longer stall for time. Levy wants to piggyback a new Dilworth Plaza onto the renovation - partly to help SEPTA solve some space problems and partly to tap into federal transit money. He estimates the plaza's cost at about $45 million.

The result is that underground transit is driving the design of the above-ground plaza, which is being overseen by KieranTimberlake and Olin.

The centerpiece would be a skylit, concourse-level waiting room that would provide access to subway lines. A long, thin glass headhouse on 15th Street would create a grand entrance to the waiting area. It would be bookended by a matching cafe.

The Kling design was also a response to transit. Its elaborate composition exists partly to mask the stairs and light wells that serve the concourse.

Levy's goal is to eliminate Kling's excess: the multiple level changes, useless outdoor "rooms," controlling granite barriers, and sunken light wells.

In their place, he would cover the plaza with a flat cap that slopes gently from 15th Street to meet City Hall. This smooth canvas would be divided in half, with earth and lawn on the south side, water on the north. The water feature would be one of those high-tech, inch-deep fountains that can be easily converted to a skating rink or public gathering space.

Those changes would soften the harsh plaza, making it more of a playground for the city, on the model of New York's Bryant Park. The downside is that the two long pavilions would occupy almost the entire length of 15th Street.

That arrangement bears a striking resemblance to the scheme Olin developed for Independence Mall. During the planning there, we were regaled with promises that the skinny pavilions bordering Sixth Street would be nearly invisible and allow clear views into the park. The reality is that the city has to look at the backs of buildings, and that the park is set off by a formidable wall.

Or consider the Comcast plaza and the problems the designers (Olin, again) had in siting the restaurant. Unless you're lucky enough to eat there, you're stuck looking at the back door from the street.

Even if the Dilworth Plaza pavilions were to achieve the promised level of transparency, it's hard to believe they wouldn't impede our views of City Hall. Rather than paying to build those big glass structures, the plaza might be better off with a modestly sized food kiosk, on the scale of Levy's new Cret Cafe on the Parkway.

The same goes for the oversized transit headhouse.

Yet in some ways, Levy's ideas for the concourse are the most intriguing. His proposed waiting room offers SEPTA a relatively easy solution for making its subway plaforms handicapped-accessible. The designers would also have an opportunity to eliminate the messy warren of corridors below the plaza and replace them with a straight walkway linking the Municipal Services Building tower and the Broad Street Concourse.

As attractive as those improvements sound, the underground portion needs more study, especially the proposal for a gracious subway waiting room. Unlike suburban commuters whose lives revolve around a fixed schedule, subway riders usually rush for the platform. And there are already plenty of concourse "rooms" west of 15th Street that cry out for upgrades.

None of this is meant to suggest that remaking Dilworth Plaza isn't a worthy cause. Levy has submitted an ambitious first draft for the space. Now, let's see the second.

Contact architecture critic Inga Saffron at 215-854-2213 or isaffron@phillynews.com.