He took the elevator to the top. Toys were scattered around the rooftop kindergarten. The swimming pool was empty. He could see, far below, an airdrome already abandoned, littered with passenger stairs. He thought he could hear bombardments in the distance. For one night, he had his pick of virtually anywhere and anything in the towers. He was a king in a disintegrating republic.

These things both did and didn’t happen. Paris would indeed fall to invading German forces on June 14, 1940, and thieves surely did prowl that dark confusion. But Le Corbusier’s proposal to replace the city’s central districts with tracts of high-rise towers was never realized. Those visions of the early 1920s, known as Ville Contemporaine, Plan Voisin, and Ville Radieuse slipped into an alternate time line, one our version of Paris, and the world, will never see.

Citizens would do well to revisit these unbuilt cities, to let their alternate histories roll around in the head. People might return to unbuilt architecture as inspiration for the future, but unrealized cities offer more than just old ideas renewed. They also remind people of how the world, so solid and certain to us now, could easily have been so different.

Though unbuilt, Le Corbusier’s proposals would go on to influence others. They inspired Lúcio Costa, Roberto Burle Marx, and Oscar Niemeyer’s urban plan for Brasília, a planned capital that seems refined to the point of being nearly inhuman. By contrast, in 1932 Frank Lloyd Wright designed his unbuilt suburban utopia, Broadacre City, in a disgusted response to Le Corbusier’s proposed Parisian grid (he called them “feudal towers ... with no life in them”).

From the beginning, Le Corbusier knew his original plan to house 3 million Parisians would have a polarizing effect. “The shock of surprise caused rage in some quarters and enthusiasm in others,” he noted. His projects contained good ideas and bad. The prospect of bulldozing vernacular architecture, obliterating streets, and displacing communities were rightly derided as philistine. But Le Corbusier’s stated intentions always offered what he saw as healthy solutions to pressing questions of the time, from slums to urban sprawl, traffic to population density. Shoddy imitators proceeded to desecrate the concept of living in the sky. Others, ignoring his careful consideration of zoning, destroy urban tracts without long-term plans. But some elements of Le Corbusier’s plans have aged well—his emphasis on green spaces as “the lungs of the city”; a serviced apartments approach (with childcare, laundry, exercise, and cuisine provided in-house) that is now omnipresent in luxury if not egalitarian circles; his romanticism of aesthetics, and his obsession with access to light and air: “As twilight falls, the glass skyscrapers seem to flame.” In their haste to condemn and forget the failings of the Plan Voisin and its kin, urbanists risk throwing away the lessons it offers in how to live rationally and even poetically.