Between 1880 and 1920, Fifth Avenue's stretch from 59th to 96th Street transformed into the wealthiest residential corridor in America. Industrial titans built 65-room limestone palaces with ballrooms for 400 guests, private art galleries larger than most municipal museums, and servants' quarters occupying entire basement floors. Andrew Carnegie's 64-room Georgian Revival mansion at 91st Street required a staff of 27 to operate daily. The Frick family's palazzo at 70th Street housed Henry Clay Frick's Renaissance masterpieces behind wrought-iron gates imported from a Tuscan villa. Cornelius Vanderbilt II commissioned a French château at 58th Street with interiors by the same craftsmen who restored Versailles. Today, Fifth Avenue's surviving Gilded Age estates offer the most accessible window into Manhattan's era of unapologetic opulence. While many original mansions succumbed to 1920s apartment tower development, a curated selection now operates as house museums preserving intact Gilded Age interiors. The Cooper Hewitt Smithsonian Design Museum occupies Andrew Carnegie's original residence at 2 East 91st Street, where visitors walk the same oak-paneled library Carnegie used for evening reading and climb the mansion's original grand staircase. The Frick Collection maintains Henry Clay Frick's art-filled residence largely as he left it in 1919, with Vermeers and Rembrandts still hung in domestic-scale rooms. Architectural historians credit these Fifth Avenue estates with establishing Beaux-Arts classicism as New York's signature style. Architects Richard Morris Hunt, McKim Mead & White, and Carrère and Hastings designed facades referencing Loire Valley châteaux, Roman Renaissance palazzi, and Georgian manor houses, creating an eclectic streetscape that signaled cultural aspiration as much as financial power. Interior spaces employed European craftsmen for hand-carved walnut paneling, coffered ceilings with gold-leaf detailing, and marble imported from the same Carrara quarries Michelangelo used. Most mansion tours in NYC focus on these architectural details alongside the social history of the families who commissioned them, contextualizing each estate within the broader narrative of American industrial wealth and its expressions in domestic architecture.