Siena's cathedral was built in the 12th century, and was the pride and joy of the city. Until, that is, Florence embarked on its building program and came up with a larger cathedral. This prompted the Sienese into the crazy idea of building the world's largest cathedral, by turning its existing cathedral into just one wing of a new super-cathedral. A set of enormous walls was then built to start Siena's Guiness record book entry, but eventually the plague made it impossible to continue with the oneupmanship. So the cathedral stayed pretty much as it was, with a couple of giant walls sticking out one side. Regardless of the addition, it's considered one of the most beautiful churches in Italy.