Brad Fiske, the KlingStubbins architect who landed the plum assignment of designing what could be the last apartment tower on Rittenhouse Square - as well as a great modern skyscraper - has tacked 40 different renderings of the project to his office walls. And that's just since August.

Some of these variations are inspired by iconic sculpture, like Brancusi's Endless Column and Louise Nevelson's collages. Others take their cues from the latest architectural icons. Fiske has mined a coffee-table book's worth of cool looks, from Norman Foster's faceted Hearst Tower in Manhattan to the bombshell curves of his Swiss Re building in London to the improbable loop of Rem Koolhaas' Beijing headquarters for China's state-run media conglomerate, CCTV.

Scanning the forms and styles on parade in Fiske's conference room, you suspect that the architect is really just playing around. The designs are too bulky, derivative and besotted with their own shape to rate inclusion in the delicate urban quilt that is Philadelphia. But after three months of intense negotiations between the developer, Castleway Properties, and city planners and neighborhood representatives, decision time is fast approaching.

Castleway paid $36.7 million for the Walnut Street site on the square's northwest corner, rescuing it from the low expectations of the Philadelphia Parking Authority, which aspired to fill it with concrete parking decks and a few movie screens. The startling sale price for the 0.83-acre parcel confirmed what real estate experts already knew: This is the region's most desirable residential building site.

The price also puts pressure on the developer to wrap up talks with the neighbors and get hopping on zoning approvals. But there is a civic interest, as well as a financial interest. Rittenhouse Square is the closest thing Philadelphia has to a town green, and what goes up on Castleway's property will have the eyes of the whole city on it.

Having computer-tested these eye-popping designs, Fiske and Castleway are now closing in on a massing scheme for height and bulk that is rectilinear and svelte, and more in the style of Kling's recent buildings. The L-shaped plan shows a 220-foot-high hotel fronting on Walnut Street, west of 19th Street, and a 525-foot-tall condo tower running along Sansom Street.

It's too early to talk about the exterior architecture, but not too soon to be concerned about the organization of the complex parts. The plan violates several cherished zoning rules and requires closing part of Moravian Street. Before the architects settle on a snazzy profile, or choose between material finishes like perforated copper or limestone, they need to deal with the usual suspects of parking, driveways and history.

Dublin-based Castleway, which is one of the backers behind Santiago Calatrava's 2,000-foot-high Chicago Spire, has gotten off to a good start. Its plans show underground parking, ample ground-floor retail, and a potentially terrific strip park running between Walnut and Sansom.

For the Center City Residents Association's negotiators, however, the monster issue is the condo tower's height. Castleway wants to upgrade the site's zoning from the current C4 to C5, the category that allows the tallest buildings.

City planners are likely to oppose the extra height and density because they see 19th Street as a dividing line, the point where Center City's high-rise core begins its descent to lower, gentler residential neighborhoods.

Castleway says it would use the zoning change to put 10 extra stories, or 120 feet, on the condo tower, for the total of 525 feet. By comparison, the nearby Rittenhouse Hotel crests at 335 feet, and 10 Rittenhouse will come in at just under 400 feet. Castleway has promised that its hotel will stand the same height as the adjacent Rittenhouse Plaza, 220 feet, to create a pleasing, uniform backdrop for the square.

Given Castleway's urban-friendly gestures, it seems a shame to get hung up on the height of the condo tower, which would be set back nearly a full block from the square. The urgent issues are on the ground. But they need to be resolved before Councilman Darrell Clarke introduces the planned zoning change.

For me, the problems begin with the organization of the retail spaces and driveways. Since the condo tower is a Sansom Street building, Fiske distributed much of the retail space there to enliven that dead block, which faces the back of the sadly decaying Boyd Theater. Meanwhile, he located the main driveway to the underground garage on 20th Street, a thriving retail corridor.

He's got things backward. That four-lane driveway, which includes an access ramp for trucks, would slash a 36-foot opening in the corridor. It's unrealistic to think the 1900 block of Sansom can be so easily salvaged. But it would be too easy to damage 20th Street. Better to shift at least some driveway lanes to Sansom Street.

More tricky is the hotel driveway proposed with an entry off Walnut Street. Because the sidewalks there are full of people, it's tempting to take a hard line and argue that arriving hotel guests should get out of their cars at the curb. But Walnut Street is a heavily trafficked through street, plied by four SEPTA bus routes. The image of hotel guests unloading their cars on Walnut Street is scary. Fiske says he would limit the driveway's impact by making it one-way and treating it as part of an adjacent park.

Every move in this project has multiple consequences. The driveway would require the demolition of two of the three historic structures on Sansom Street, which became a cause célèbre in the long legal battle against the municipal parking garage once proposed for the space. Yet those buildings are also a problem for the garage, Fiske says, because they block its sloping ramps.

What a choice for Philadelphia: The city finally gets a developer willing to shell out the extra bucks for underground parking, but two historic buildings have to be sacrificed. At least they are the less interesting ones. The tiled Rittenhouse Cafe would be restored. Castleway has also made an unprecedented offer to compensate by renovating two other historic buildings nearby.

It would be nice to see more details of Fiske's design. But even in its fuzzy beginnings, it shows signs of being an important building that will make Philadelphia proud. The challenge now is to solve the down-to-earth urban concerns, while still allowing room for the architecture to soar.

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