Peeking over a railing, I look down into a heavily wooded ravine where beefy buildings in concrete curve around courtyards and wrap two ponds vaulted by copper- roofed bridges.

This hidden castle is the Crystal Bridges museum, in Bentonville, Arkansas.

It houses a historical survey of American art collected in just a few years at astonishing cost at the instigation of Alice Walton, heiress to the Wal-Mart Stores Inc. (WMT) fortune.

Passing a stainless-steel tree sculpture by artist Roxy Paine, I cross a bridge to an elevator tower and descend three floors, where one of the bridges that give the museum its name opens up in front of me.

It’s a tour de force. Deep beams of yellow Arkansas pine vault overhead, rising and swelling outward like the belly of a particularly elegant whale. Light filters in from slots overhead. Outwardly slanting glass walls held together by elegant pipes, ball-joint fittings and cables pick up reflections from the wind-riffled surface of the ponds.

No art could compete with this spectacle. The vast space is used as a restaurant called Eleven and for parties.

With Walton, 62, architect Moshe Safdie chose a wooded streambed in a 120-acre swath of woodlands the family owns just north of the headquarters of the retail giant her father built. Safdie dammed the streams to form the ponds. Walton chose the generically bucolic name Crystal Bridges.

Sensual Safdie

Safdie, 73, doesn’t need to strain for grandeur; bold forms come to him naturally. He shaped balconied and arcaded gallery suites into broad crescents with draped copper roofs to echo the slope of the land. He tones down a characteristic bombast in recognition of the lovely ravine, and he achieves a lyricism and sensuality rarely found in other projects.

Tightly wedged, the 200,000 square feet (18,580 meters) of pavilions stare narcissistically at each other over the water. The Walton family has threaded the surrounding forest with pleasurable trails dotted with sculptures including a “Skyspace” installation by James Turrell.

As you enter the first two gallery suites devoted to art from the colonial era to the end of the 19th century, you encounter a collection that’s esthetically conservative and methodically assembled, including two famous portraits of George Washington (Charles Wilson Peale and Gilbert Stuart) and great painters of grand, idealized nature like Thomas Cole, Frederic Church, and Asher Durand, whose 1849 “Kindred Spirits” cost $35 million.