Most historians of baroque architecture agree that it is theatrical in some way, without describing in detail the festivals and performances that inspired its design. Yet festivals structured public life in most early 18th century European cities, and were a central aspect of what is commonly called the Baroque Unity of the Arts. As festivals became a permanent aspect of city life, the character and morphology of buildings changed too. Facades became explicitly stage-like, transforming civic spaces into theatres. The universal motifs of the niche and portico reveal that baroque buildings were...