In his story “The Secret Room,” the French novelist Alain Robbe-Grillet describes an interior so ornate as to defy description, whose very dimensions are shadowy but “whose considerable space must in reality extend all around, right and left, as it does toward the faraway browns and blues among the columns standing in line, in every direction.”

The Veterans Room at the Park Avenue Armory — completed in 1881 by the then-brand-new dream team of Louis Comfort Tiffany, Stanford White, Candace Wheeler and Samuel Colman — may be as close as any room in New York City comes to such beyond-words fantasia. Depending on which inch you’re looking at, it’s Islamic, Chinese, Greek, Celtic, Egyptian, Persian or an indefinable mélange of them all, on which no expense was spared. It’s never been a secret (the year it opened, this newspaper declared it “undoubtedly the most magnificent apartment of the kind in this country”), but for decades its glories have been concealed beneath bad repairs, inadequate lighting, brown paint and a patina of Gilded Age cigar smoke.

On Monday it reopens to the public in all its historical swagger after more than a year of work, at a cost of more than $8 million, by the Swiss architecture firm Herzog & de Meuron, which is overseeing the restoration of the armory, built as a bejeweled boys’ club by New York State’s Seventh Regiment of the National Guard, the blue-blooded militia that was the first to respond to Lincoln’s call for troops in 1861.

Since 2009, the armory, a once-crumbling landmark, has transformed itself into one of the world’s most sought-after venues for performance, music and supersized art projects. And in a sense, the Veterans Room, of all the Armory’s opulent reception rooms (three others have been restored so far), has the deepest spiritual kinship with a work of contemporary art, the feel of an installation by a young collective whose members were reacting to one another and making it all up as they went along.