"It is from their foes, not their friends, that cities learn the lesson of building high walls."
Masonry has multiple effects. Immediately, it allows your Workers to build Quarries on Stone and Marble resources, granting several production bonuses. Additionally, Masonry enables the construction of Walls and the mighty Pyramids world wonder. Later, the Great Lighthouse and Aqueducts can be built with Sailing and Mathematics respectively.
The early dwellings of most tribal cultures were built of materials that were readily available and easy to work with, such as bricks of clay and mud. As tribes gave up their nomadic ways and settled the first cities, they soon found that they had a need for more permanent and durable structures. The skill of masonry was developed to fill this need.
The earliest stone structures consisted of little more than rocks that were stacked atop one another to form crude walls. Artisans soon began to square off the rocks, forming them into regular shapes and stacking them. These early examples of masonry used no mortar; the weight of the stacked stones provided overall strength and stability. Without the use of mortar and knowledge of architectural techniques, masonry was used primarily for simple structures such as fortifications. However, some examples of early masonry are quite spectacular, including the Great Pyramids in Egypt, which are still standing after thousands of years.