This document discusses the evolution of cities and argues that the 21st century is the century of the city. It outlines challenges facing modern cities like pollution, congestion, and population growth. It then introduces the concept of a city platform that treats a city's data and services as an open platform. A city platform could help cities collaborate to solve common problems and create marketplaces by establishing a network effect. The document advocates for a holistic smart city approach using a city platform to meet citizen needs and curate a digital ecosystem through multi-stakeholder governance. Significant progress has already been made in developing business models, technical capabilities, and open APIs to support the vision of a city as an open platform.