The Mendelssohn Club of Philadelphia is only blocks away from its usual Kimmel Center environs, but artistic light-years removed from

Carmina Burana

. On this Monday evening, under the merciless glare of high-wattage fluorescent lights, the chorus is both singing and maneuvering through various formations, along with more intricately choreographed dancers inside the Armory of the First Troop Philadelphia City Cavalry.

About 50 yards away, on a high platform, music director Alan Harler conducts his singers with a flashlight. He could have asked for a safety railing - the Kimmel Center supplies such things. "But I didn't want people to think that I was chicken," said the patrician, white-haired conductor. He has collaborated with the Leah Stein Dance Company on Battle Hymns, a new work by Pulitzer Prize-winning composer David Lang, premiering Saturday.

The fact that Harler and company are so far out of their element means they're in the Hidden City Festival - a month-long series of site-specific events in notable but often derelict Philadelphia buildings. Usually a haven for tanks and humvees, the Armory is luxurious compared to the project's Plan A: the long-abandoned Frankford Arsenal in Bridesburg. "There were way too many problems," said Harler. "It had no plumbing, lots of broken windows, pigeon droppings from 400 years. . . ."

And a location problem. Unlike the arsenal, audiences won't have to hunt for the Armory, which is nestled into 22 S. 23d Street. "It's quite beautiful during the day. The light is wonderful," said choreographer Stein, who requested daylight performances, at 4 and 6 p.m., this Saturday and next. "Maybe because there's a war going on, I wanted a connection to the outside. I don't want the piece to feel like, 'Now, we're in the theatrical world.' "

Stein is no stranger to creating site-specific choreography and would seem an unlikely collaborator for the usually indoor Mendelssohn Club and its repertoire of big-chorus classics. Nonetheless, Stein and Harler were seen embracing after rehearsal; they've worked together often. Harler's less-visible concerts explore the dance-oriented music of Meredith Monk, and the Mendelssohn Club, long familiar with new music, has commissioned 43 works. Thanks to experience in musicals, chorus members such as John Dinsmore are comfortable with simultaneously moving and singing.

"Most of the group, I'd say, are plant-and-scream singers. But they're taking it in stride," he said. "It's such a cool piece, and it's just nifty to be able to do something different. I told some of the people in the Crossing [choir] what we're doing, and they said, 'What? Are you kidding?' "

The music is built from extremely simple elements - composer Lang is a descendant of the repetition-based minimalists - but isn't necessarily easy. "Small changes are happening all the time. The text and rhythm is shifting all the time in a way that's mathematical," said Harler. "It has enormous emotional impact. There's something medieval about it. It's the same impact that I get from listening to a Dufay Mass."

The music is also no easier to memorize than the multilayered Dufay. Choreographer Stein hoped singers would have both arms free; instead, they will have scores in hand - though the disappointment is dwindling as all parties concerned are taken in by the unfolding felicities of the music. "The mind that put this together is incredible," said Harler, "just incredible."

Similar statements came from members of the 2008 Pulitzer Prize jury that honored Lang's choral work The Little Match Girl Passion, which juxtaposes the Hans Christian Andersen tale with texts from Bach's St. Matthew Passion, now published on disc by Harmonia Mundi. Such recognition also means that Lang, one of the few composers to emerge from Manhattan's downtown, cutting-edge Bang on a Can Festival, is in such demand that he could only participate in preliminary site explorations.

The Bridesburg arsenal, though less than viable, had a lasting impact on him. Its key role in the Civil War inspired Lang's choice of texts - including Abraham Lincoln, Stephen Foster, and letters written from various Civil War fronts - and the military nature of the piece.

From the outset, choreographer and composer agreed that Battle Hymns would not be political. "The Civil War was different from the Revolutionary War, which was so clearly 'us versus them.' There's something inherently introspective about the Civil War, because it's about us. I thought about what people's ordinary lives were like," Lang said by phone from his SoHo loft. ". . . I'm not a pacifist. The fact that I'm completely against the war we're currently in does not mean that I'm not open to the possibility that we need to have armories to protect us."

Stein admits to apprehension about staging a piece in a working armory. Its historic distinction - housing the first-ever cavalry unit in the United States - meant she was in the belly of the beast. Then she discovered that her helpful contacts within the armory shared her anti-Iraq war sympathies and were quite attuned to the performing arts. Still, she says, the experience is "intense and complex," given how many people she knows who have suffered directly from war.

"I'm a little bit in awe of the music and wondering if I can meet it or match it, and breathe into this music with this site," she says. "It's an enormous project. Sometimes, I feel that I'm just scratching the surface."

No wonder Lang doesn't apologize for his music's difficulty. "If somebody is going to commission me, they're doing so because they found something that resonates with them in my work," he said. "It's not about a great performance, but whether or not they get to access the [artistic] opinion [of the piece]. I like the idea of giving this hard piece to these performers. And then afterwards, we'll all go out and drink too much, and it'll be great. . . ."

Contact Inquirer music critic David Patrick Stearns at dstearns@phillynews.com.

"Battle Hymns" will be performed at 4 and 6 p.m. Saturday and June 20, at the Armory at 22 S. 23d St. Tickets: $15-$20. 267-597-3808 or www.mcchorus.org