When urban density reaches a certain point, verticality becomes a crucial attribute of the city.
In such a city, all components of urban design including circulation, land uses, open spaces, ecologies, and human activities are distributed in a different pattern and their relationships mutate. As we can see in some of the world's megacities like Hong Kong, Shanghai, Tokyo, Seoul, and New York, the floor area to plot ratios go beyond 1:12 and residential densities exceed 400 persons per acre. With such intensity, the planning area of a city is no longer its surface but the entire built-up area and the potential buildable vertical space above. In such cities, transportation, social programs, and open spaces are highly integrated into a system that stretches from underground to the top of buildings. Given such complexity, we can no longer simply focus on the site plan and layout of buildings but should examine the city as a three-dimensional matrix for urban design solutions.
In contrast, Vertical Urbanism is not a static form; rather, it represents a dynamic and adaptable strategy of urban design and development. This strategy moves away from the Modernist notion promoting tall buildings as dominant urban typology and toward the exploration of physically interactive and socially engaged forms, which address the city as a multi-layered and multi-dimensional organism. It is informed by contemporary urban systems ranging from underground mass transit to futuristic urban farms and conceptualizes the city as a holistic organization of infrastructure, space, and ecology in a three-dimensional framework. Density, complexity, and verticality are three keywords to describe the characteristics of Vertical Urbanism. Density indicates the concentration of population, programs, and developments, as well as the intensity of resulted social interaction. Complexity refers to the multiple interconnected urban systems forming the backbone of a contemporary metropolis' operation. Verticality addresses the multi-dimensional spatial network responding to such density and complexity to provide an urban environment that is efficient, sustainable, and socially engaging. source -Vertical Urbanism: Re-conceptualizing the Compact City