Final answer:
De Botton uses the phrase "a carnival of architecture" to criticize the negative impact of modern architecture on historic landmarks.
Step-by-step explanation:
In the context of the quote you provided, when de Botton writes, "we suffer from a carnival of architecture," he is expressing his disapproval of modern architecture and its impact on the city of Paris. He is describing a hypothetical scenario where an absurd and imposing tower dominates the city, dwarfing and overshadowing its historic and cultural landmarks. This metaphorical carnival of architecture represents the negative consequences of modern architectural designs that disregard the harmony and preservation of historical and cultural values.