Before Britain ruled with waves, it was the empire of Spain that was known as the empire on which the sun never sets. For a century or so it even joined with Portugal to be the first and only empire of land and sea across multiple oceans that was built with pre-industrial resources. But it declined. When? How? Why? And should we remember these empires not by images of conquistadores and the Spanish Inquisition, but by walking through the Botanic Gardens of Madrid?