Cities such as Siena, Paris, Edinburgh etc are, in most part, made up of quite ordinary buildings. However, these buildings work together in an orderly fashion to create something much greater than their individual contributions.
Enclosure is at the heart of this order. A street or space feels enclosed when urban blocks are built in proportion, they provide continuous walls that clearly define spaces at the back of the pavement. For instance, buildings should be at least as high as the street is wide and blocks should be made of buildings that join together at the back of the pavement.
As well as providing a sense of coherence, urban grain, combined with street trees and green space, can reduce exposure to extremes of weather and bring about hospitable microclimate.