It's known, locally and in numerous architectural archives, as the "Castle House," but you'd never find it if you went in search of a castle-like structure along New London's waterfront Pequot Avenue.

Instead of an ornate and massive bastion of tradition, the so-called Castle House is a sleek, airy, one-level showcase of modernism, its defining feature a cypress butterfly ceiling that appears capable of flight. The only traditional element is the 89-foot-tall 1801 Harbor Lighthouse next door.

The name actually comes from the Castle family, for whom the house was designed in 1962 by Ulrich Franzen, one of the era's leading modernist architects. House & Garden magazine featured it the same year, calling it a work of art that "kindles excitement," and again in 2005, after a renovation by its current owners, dubbing it "one of the finest flowers of high modernism's mannerist period."

Thanks to the hit AMC television series "Mad Men," set so far in 1960-65, most every aspect of that period is enjoying a robust revival of interest. And this Pequot Avenue property, built for a prominent businessman and his family, would make a dazzling set piece for the series.

It's easy to imagine the "Mad Men" and their wives or lovers downing martinis on the circular, open-air dining platform that's part of a boat dock. Back inside, someone might open one of the lustrous bird's-eye maple cabinets to reveal a bar with sink, or a carousel for folding chairs.

The house, currently offered for sale through Heritage Properties for $1,995,000, is notable as one of four private homes designed by Franzen in Connecticut. Along with two others of similar vintage, one in Essex and one in New Canaan, the New London house made it into the Architectural Record.

The fourth house, far grander in size, was built in Greenwich in 1994.

When Franzen designed the New London house in 1962, big changes were afoot in politics, the American family, fashions, art and music, and home décor. Christopher Wigren, architectural historian at the Connecticut Trust for Historic Preservation in Hamden, says this was also an "important, very fruitful" period for modern American architecture.

Modernism, with its clean lines and focus on function, wasn't new, having emerged in Europe in the 1920s and '30s. But prior to the late 1950s and early '60s, American houses in this style were obsessively practical.

"A lot of the modernists had gone to the same schools, were doing the same thing," Wigren says. In the 1960s, "the aesthetic was more individual, experimental."

Franzen, who turns 90 next year, hasn't lost that individual spirit. Contacted at the home he designed for himself in Santa Fe, N.M., he is unconcerned that his particular aesthetic was never mimicked en masse.

"I never intended to clutter the landscape," he says. Rather, he hopes that his ideas have been "fruits on the trees," ripe for picking by discerning others.

A 'Pleated' Ceiling

Wigren says the "pleated look" of the double-arced cypress ceiling in the New London house reminds him of origami. He hasn't seen the house in person, but the ceiling looks to him on paper as if it was meticulously folded, then opened up to reveal an elegantly fanciful organic shape.

Set on a rock outcropping where the Thames River fans out into Long Island Sound, the house is a prime example of Franzen's particular skill at integrating buildings into difficult landscapes.

It was built of bricks, steel, plaster and glass, on a platform several feet above the highest recorded hurricane tides.

The swimming pool, inset atop rocks at the water's very edge, was meant to serve as a buffer to extreme weather, and the house's landscaping was designed to counter its exposed setting.

Franzen believed a house should reflect the sensibilities of the era, and this one's second and current owner admits the architect wasn't pleased when some elements of his original design were lost in the 21st-century renovation.

Chief among them were the mostly enclosed galley kitchen, one feature of 1960s style that practically no one wants to resurrect, and a master bedroom suite with floor-to-ceiling glass looking out onto an enclosed courtyard rather than, as now, the far more dramatic vista of the Sound.