Through multiple earthquakes, misguided urban renewal schemes and changing economic conditions, the Ferry Building has stood at the foot of San Francisco’s Market Street since 1898. In his book, “Portal: San Francisco’s Ferry Building and the Reinvention of American Cities,” John King, the urban design critic of the San Francisco Chronicle, tells Unfrozen what we can learn from the indefatigable icon, and what that might mean for the future of downtowns in this uncertain era.