For a penthouse, the top of One Liberty Place hasn't seen much action.

First, it was a Cigna corporate cafeteria. Then, it sat vacant. Soon, though, the space - and, more important, the views - will be accessible to anyone willing to pay $19 a ticket.

The One Liberty Observation Deck, to open in November, represents an investment "north of $10 million," said general manager Evan Evans. He expects it to attract half a million visitors per year.

To his employer, the French company Montparnasse 56, it was an obvious choice.

"You try to figure out the highest and best use of real estate," he said, "and observation decks can generate an extraordinary amount of revenue through a relatively small amount of space."

As quaint an attraction as such a vantage point can be, there has been an international boom in construction of tricked-out versions: In Chicago, the Willis Tower invites viewers to experience "The Ledge," while the John Hancock Center invites them to "Tilt!" over city streets below. The recently opened One World Observatory in Lower Manhattan is expected to bring in fully one-quarter of the revenue for the World Trade Center.

But, until now, the only observation deck in Philadelphia was the tiny walkway atop City Hall Tower - and for only 10 minutes at a time.

So why did it take so long for us to catch up?

Perhaps it's that developers here are reawakening to the value of a great view. Other indications of that: the repurposing of underutilized sky-high spaces (like restaurant R2L, opened in 2010 in Cigna's old mail room on the 37th floor of Two Liberty Place) and the inclusion of elevated attractions in new construction (like Comcast Technology & Innovation Center, which will include a top-floor restaurant).

Observation decks and penthouse restaurants mostly weren't part of the 1980s and '90s skyscraper boom (though Penn Mutual invited tourists to view Independence Mall from its 19th-floor deck, until it closed in 1985).

Mickey Rowley, co-owner of Top of the Tower, the banquet hall atop Three Logan Square, has had a bird's-eye view of the newfound interest.

A few decades ago, he learned that Bell Atlantic, then the building's anchor tenant, was barely using its conference space on the 50th and 51st floors. He was working at a hotel nearby and struck an informal deal to use the space for weddings and other events.

In 2013, when Brandywine Real Estate Trust bought the building, Top of the Tower was no longer an afterthought. The company invested in renovating and expanding the facility, opening up long-shuttered windows.

"It was amazing, the views that were boarded up for 20 years," he said. Since then, he has been adding more public-facing events. In March, he launched a weekend Skybrunch. Next, he plans to add a Skybar. The views are always free, he said, though brunch is $50.

He conceded: "I'm in the real estate business, not the food business."

Brandywine is also including public and private viewing points in its Cira Centre South expansion: There's a 415-foot-high veranda planned for the new FMC Tower, for building tenants only, and there's a publicly accessible area called Cira Green that will be "an incredible floating sky park that's almost 100 feet above street level," said Steve Rush, vice president of leasing for Brandywine.

"There's absolutely a trend toward creating a unique complement of amenities," he said. "Whether an observation deck or a restaurant or a public park, it really sets you apart and can add to the value of the building."

There's a similar concept at work at the Comcast Innovation & Technology Center, which will be topped with a Four Seasons hotel and a restaurant. That Philadelphia now is seeing complicated multiuse projects like Comcast and FMC, along with retrofits like One and Two Liberty, reflects a maturing market, said Liberty Property Trust senior vice president John Gattuso.

"If anything, the fact that you see vertically integrated uses ... speaks to the emerging sophistication of Philadelphia's marketplace," he said. "These are much more complex, sophisticated buildings than one could have envisioned doing 10 or 20 years ago."

The infrastructure involved in adding an observation deck to a skyscraper is almost as complicated, Evans said: It was a significant victory to be able to get visitors to the top in a single elevator ride.

From the One Liberty space, currently a dusty construction zone, he pointed to the features that make for a good observation-deck site: architectural points of interest, changing topography, and bodies of water.

"Water is always something people love seeing from above," he said.

In case that's not enough, though, One Liberty will reflect the 21st-century trend of viewing decks as theme parks: There will be an entryway lined with Philly-themed photos and signage; a lighting-effects show that tells the story of Ben Franklin; a Franklin mural; and a giant Franklin statue, featuring his feet on the ground floor and his head upstairs on the viewing deck. There also will be touchscreen maps that viewers can use to zoom in on and learn about sites in the distance.

With its interactive maps and its we'll-text-you-when-it's-your-turn-in-line technology, One Liberty will be drastically different from the only observation deck currently open in Philadelphia, atop City Hall Tower.

For instance, One Liberty will be served by an elevator that can lift 25 people in 75 seconds. The deck can hold 293 people.

At City Hall, only four or five people can visit every 15 minutes. It's a two-minute elevator ride, and a 10-minute stay at the top.

"We're lucky if we can get 120 people up in a day," said Greta Greenberger, who runs the tours, which are a collaboration of the city and Independence Visitor Center.

On the other hand, "you can look up and see William Penn standing, all 37 feet of him, above your head. You really get a sense of Penn's plan for the city and how it's laid out. So it's an intimate brief experience."

In summertime, she has to turn people away. In winter, visitors are more scarce. Besides, City Hall Tower isn't climate-controlled, and tours can't be conducted in icy conditions.

Greenberger has run the tours for 23 years. How a fancy new deck right around the corner might impact her life's work is hard to say.

"I don't know what they're doing up there," she said. "But it's going to affect us for sure."

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