Cities: Skylines focuses on city-building simulation through urban planning. Players control zoning, road placement, taxation, public services, and public transportation of an area. Gameplay is open-ended, touted as "endless sandbox gameplay across massive maps." Player-determined civic policies can affect the development of the entire city or individual districts thereof.
Roads can be built straight or free-form and the grid used for zoning adapts to road shape; cities need not follow a grid plan. Roads of varying widths (up to major freeways) accommodate different traffic volumes, and variant road types (for example roads lined with trees) offer reduced noise pollution or increased property values in the surrounding area at an increased cost to the player.
Finnish developer Colossal Order had wanted to create a game with a broader scope than its transportation-focused Cities in Motion games for some time, but could not initially secure funding from publisher Paradox Interactive. Cities: Skylines was announced by Paradox on August 14, 2014 at Gamescom while in the alpha stage of development. The announcement trailer emphasized that players could "build [their] dream city," "mod and share online" and "play offline"—the third feature was interpreted by journalists as a jab at SimCity, which initially required an Internet connection during play. Skylines uses an adapted Unity engine with official support for modification.