This soaring “Cathedral of Commerce” cost Frank W. Woolworth $14 million worth of nickels and dimes in 1913. But no amount of money could get visitors into its famed interior of this private office building—until now. The great-granddaughter of architect Cass Gilbert started offering tours in late 2014, and now anyone who makes an advance reservation can learn about this architectural wonder, the tallest edifice in the city until 1930, when it was surpassed by the Chrysler Building. (At its opening, President Woodrow Wilson pressed a button from the White House that illuminated the building’s 80,000 electric light bulbs.) Called the “Mozart of skyscrapers” by architectural critic Paul Goldberger, the neo-Gothic architecture is festooned with spires, gargoyles, flying buttresses, vaulted ceilings, 16th-century-style stone-as-lace traceries, castlelike turrets, and a churchlike interior—all of which you’ll examine in the company of knowledgeable, hand-picked guides (many noted architectural historians in their own right).