OVER THE COURSE of its century-plus history, the floor of the Armory of the First Troop Philadelphia City Cavalry has undoubtedly been crossed by scores of marching feet. These past few weeks, however, have surely marked the first time those feet have marched with musical rather than military precision, as choreographer Leah Stein has put 85 singers from the Mendelssohn Club and eight dancers from her own company through their paces in preparation for the premiere of "Battle Hymns," part of the month-long Hidden City festival.

The fact that this formation is singing together in gorgeous harmony doesn't make them any less imposing as their march approaches, and the four of us seated in the middle of the Armory - standing in for an eventual audience of 250 - are sent scrambling, folding chairs in hand, towards the camouflaged military vehicles at the back of the warehouse-sized room. The echoes and incongruities with the space's history, evident even at this early rehearsal, are precisely the sort of interaction that Hidden City was designed to create.

"It's really about the recontextualization of contemporary work," explained Thaddeus Squire, artistic executive director of Peregrine Arts, the presenting and producing organization behind the festival.

"This work, which is often very difficult for the broader public to access, can be itself in these spaces and accrue the benefit of context from the narrative and the memory of the space. So there's a story next to the work, but the work itself doesn't have to tell a story."

Throughout June, Hidden City aims to bring to light nine of the city's buried architectural treasures through a series of performances and installations. The pieces range from a musical performance and film/video installation in South Street's abandoned Royal Theater to a print installation in the Daily News/Inquirer Building. There is even a card game which allows players to construct neighborhoods from their varied resources.

Hidden City has been the primary focus of Peregrine Arts since its founding in 2006. The festival combines Squire's interests; formerly the artistic director of the Relache new-music ensemble, he also has a background as a "historian and philosopher of science" - as well as some slightly less reputable, though valuable, experience.

"When I was growing up," Squire said, "a friend and I used to break into estates and mansions on the Main Line and rehab them. We were caught by the Lower Merion police on several occasions. So I've always had an interest in going into spaces like this."

Hidden City emphasizes sites from the 1800s, a period which Squire says tends to get lost in Philly's focus on its Colonial-era heritage.

"Philadelphia is very proud and interested in its history, but it's also very good at hiding a lot of its history, particularly its 19th-century history. But the time that actually built this city was the Industrial Revolution. So ironically, we've sort of forgotten the city's literal heyday, economically, socially, culturally and otherwise."

"Hopefully Hidden City will ask more questions than it will answer," said Jay Wahl, Peregrine's managing producer, looking down on the Met's stage. "People will sit here and be moved by the dancing and start to think about what this building could be, and then start thinking about what it was, and then they walk out the door and the site is less a destination than a departure point."

The Armory, a granite, castle-like structure in the midst of Center City's concrete and steel, has a story stretching back past its construction in 1901. It was built to house the oldest cavalry unit in the United States, which had served as George Washington's personal bodyguard in the Revolutionary War, fought at Gettysburg in the Civil War, and continues as an active National Guard unit today. Pulitzer Prize-winning composer David Lang responded to the site's history with a suite of vocal pieces incorporating text from the letters of Civil War soldiers and lyrics from Stephen Foster tunes.

"I really responded to this feeling in the music of the complexity of the military as a unified force and at the same time the more personal story and experience underneath that," said choreographer Stein.

While "Battle Hymns" takes its inspiration chiefly from its site's history, other artists responded more to the aesthetics of their venues, none of which are nearly as striking as the Metropolitan Opera House at Broad and Poplar. Built in 1908 by Oscar Hammerstein (the lyricist's grandfather), the decaying 4,000-seat hall now seems like the ghost of a grand opera house, a feeling amplified by the echoing music and dancers navigating the debris-cluttered tiers during a recent rehearsal.

Brooklyn-based choreographer Wally Cardona was commissioned to create a piece in the space for Philly's Group Motion Dance Company, set to the music of composer Phil Kline, a task which he compared to making a dance on the side of a mountain.

"The place is one of the most haunted, magical, extremely powerful spaces I've been in - not as in places I've made work in, but in my life," Cardona said.

The building is currently the home of the Holy Ghost Headquarters Revival Center, a church housed underneath the theater in what was the orchestra pit - an interesting new context, and one which highlights the fact that for these sites, history is not just a thing of the past. Cardona recalls working upstairs one day when he began to hear the preachers' voices rising from below.

"It was quite overwhelming emotionally to experience this whole other world happening downstairs," Cardona recalled. "There was one part of me that thought we shouldn't be here. But then I realized what a perfect example this is for a world that I want to live in.

"Downstairs, they're fully committed to what they're doing and the role of it in their lives, and here we are upstairs, being fully committed to what we do and the role that it plays in our lives." *

Hidden City Philadelphia through June 28, 267-597-3808, for information, schedule and tickets, www.hiddencityphila.org.