Cities Online is a cross-platform implementation of the board game Settlers of Catan with all expansions, including Seafarers, and Cities & Knights. It displays the game world in 3 dimensions, with rotation, panning, and zooming, in bright beautiful textures contributed to the project freely by talented individuals from around the world. The user interface is entirely custom drawn, with crisp, clean lines and dozens of high-quality bitmaps to facilitate an intuitive, pleasing experience for users. Cities Online has an extensible plugin architecture that allows new rules and variants to be added into the game without changing an ounce of code in existing rules or variants. In fact, the core rules themselves for Settlers, and the expansions, are all plugins.