She rose above the broad panes of shop windows. The channels of streets grew deeper, sinking. She rose above the marquees of movie theaters, black mats held by spirals of color. Office windows steamed past her, long belts of glass running down... The line of the ocean cut the sky. The ocean mounted as the city descended. She passed the pinnacles of bank buildings. She passed the crowns of courthouses. She rose above the spires of churches.
Then there was only the ocean and the sky and the figure of Howard Roark. (4.20.15-9)
In the last scene of the novel we follow Dominique as she visits the construction site of Howard Roark's new skyscraper. Roark is now her husband (the male and female leads have fittingly gotten married), and the building is the skyscraper to end all skyscrapers. It is the best skyscraper ever. Dominique hops in an elevator and goes up and up and up to meet her new hubby.
The metaphor here is a bit heavy-handed, but effective. Dominique rises up above various other markers of civilization and makes her way up to Roark, the individual extraordinaire who represents a new kind of person and a new (and better) kind of society.
Roark is almost god-like here, hanging out on a massive structure that he built, literally towering above everyone else. With this image, we know that Roark won his battle and can live on his own terms. He may not have defeated the bad guys entirely—Ellsworth Toohey is still alive, after all—but Roark and Dominique and their unconventional bundle of joy (Daddy's new skyscraper) get a sort of triumphant, and even happy, ending.