CircleMUD

CircleMUD is a free, open-source MUD server codebase written in C that enables running text-based multi-user dungeon games. It provides the underlying engine for creating persistent virtual worlds in which players explore environments, engage in combat, interact with NPCs, and manage inventories through telnet or similar clients. First released in the mid-1990s, CircleMUD evolved from the DikuMUD/Merc lineage and has since become a widely used platform for hobbyist and educational MUD development.

The codebase is designed to be modular and data-driven. Designers configure areas, rooms, objects, mobiles (NPCs), shops, skills, spells, and other gameplay elements through data files and adjustable code, allowing extensive customization without rewriting core game logic. CircleMUD supports features common to many MUDs of the era, including class and race options, a combat system, quest and social mechanics, and administrative tools for moderating the game world.

Over the years CircleMUD spawned numerous derivatives and forks, and multiple major revisions (commonly referred to as Circle 3.x and related series) introduced improvements to memory management, scripting, and portability. The project has been maintained by a global community of volunteers, with ports and patches extending compatibility to modern Unix-like systems and, in some cases, Windows environments. CircleMUD has influenced later MUD engines and remains a touchstone for researchers and hobbyists studying the history and architecture of online text-based worlds.