In 1570 Andrea Palladio published his 'magnum opus' (major work), a treatise on architecture in which he explained his principles and gave practical suggestions to builders and designers. What is its title (in English) ?

The English title is "Four Books of Architecture".

Explanation:

"Four Books of Architecture" is a work written and illustrated by Andrea Palladio, one of Italy's most important architects. After being published numerous times in Venice, the book was spread all over the world, showing great works of classical Greco-Roman architecture and becoming the greatest work written by Palladio. The original title of the book is "I quattro libri dell'architettura", being translated into English as "Four Books of Architecture", which was assumed as the official title in English-speaking countries.

The English title is "Four Books of Architecture".