Architecture 

The centrality and iconicity that made the World Trade Center a target — the biggest buildings in the biggest city in the U.S., two for one — gave many a focus for tributes in the wake of their destruction, 20 years ago, on Sept. 11. From memorial flowers and candles on the Brooklyn Promenade overlooking their absence, to calls for rebuilding, to the twin searchlight beams of the Tribute in Light, to the eventual form of Michael Arad and Peter Walker’s 9/11 Memorial — where the towers’ exact footprints are rendered as eternal voids — the Twin Towers were celebrated as symbols of strength.