Inside the Armory of the First Troop Philadelphia City Cavalry on the prosaic block of 23d Street just below Market, art, architecture, and nature collided Saturday in near-mystical blasts as choreographer Leah Stein and Mendelssohn Club music director Alan Harler mustered their forces for composer David Lang's

Battle Hymns

.

Part of the current Hidden City Festival project, which highlights rarely seen landmarks from Tacony to South Philly, it will be performed again next Saturday.

Arriving in the cavernous space for rehearsal mid-week, I noticed that there was no echo, perhaps because the concrete floor was covered in black rubber.

Harler noted, "Acoustically, it's not entirely dead. And a lot of Leah's choreography takes us through the space so people hear better depending on where they are. But," he added, "the risk of trying something that isn't in the standard venue is invigorating, and we have the forces to overcome any auditory problems."

Viewing problems were harder to overcome; the audience, seated in two diagonal rows and one across, would have had better sightlines from bleachers.

"And the rubberized floor isn't any more comfortable for the dancers," Stein said. "It's all part of how we adjust to the space and budget."

When rehearsal began I saw a lot of running and lifts, but not much jumping. Harler called Stein as she walked away from making an adjustment with the singers and she did a 180-degree turn with military precision. The space's 235-year history had invaded her, in what dancers call "muscle memory."

Pulitzer Prize-winner Lang, commissioned to write the music, noted that his other choral works had been performed in standard concert halls. "What intrigues me about this space is that there will be nothing else to compare it to," he said. "And that these singers are so willing to move around like this - while singing."

In the hushed opening moment at Saturday's performance, the massive wooden doors swung wide to the street. A lone rider on a copper-colored horse clopped in, examined the waiting assembly, shrugged, turned, and rode out. Eight dancer/soldiers invaded the space stealthily from different directions. The main doors slowly opened and the 75 singer/officers strode in, scores in hand. They wore Heidi Barr's costumes - the soldiers in filthy, sweaty tatters; the officers in spiffy gray-blues.

In an interlude between Lang's five sections, Philadelphia percussionist Toshi Makihara loudly drew our attention to himself high up on the west-wall catwalk. Below, dancer Makoto Hirano planted his feet before a large jeep, as dancer David Konyk took the wheel. Hirano swung his shopping bags and did the Tank Man Tango, modeled after the man who defied the tanks in Tiananmen Square in Beijing on June 4, 1989. (Thanks to YouTube, people around the world did a networked tribute performance of the dance this year.) Stein added the section at the last minute.

Outside, several dancers ran up a walkway on the building behind, visible through the arched windows. As Hirano climbed onto the vehicle, a near-mystical blast of sunshine showered over him.

This massive undertaking required collaboration and compromise. It didn't always happen smoothly, but in the many moments that it did, like this one, it evoked a kind of reverence for the space and for the military sacrifices the entire work represented.

Konyk also dances with Group Motion Dance Company and will perform with them in Revival, Hidden City's other dance event. Group Motion director Manfred Fischbeck commissioned New York-based Wally Cardona to choreograph the piece with music by Phil Kline, their fourth collaboration. It will premiere June 24 at the Metropolitan Opera House on North Broad Street.

"It's an amazing experience to work with two such different choreographers in two vastly different spaces," Konyk said. "Here, with Leah, we're working on a flat, level surface. But at the Met we're working on multiple levels - and Wally is so radically different from Leah."

Each dancer studies Cardona's videotaped movement separately. "And we're not allowed to confer with each other," Konyk said. "So, as opposed to Leah giving us her organic, dropped-weight movement, this is you taking ownership of the movement."

By phone from New York, Cardona said what he liked about working with the Group Motion dancers "was their ability to listen, try and then do."

"When Hidden City caught wind of this collaboration," Cardona said, "we met up with the producer, Thaddeus Squire, to see several sites - all incredible - but the others were more about empty space that needed to be filled.

"The Met seemed so full already. You walk in and the show has already begun - the layers of history are very present."