Citadel is the name of a bulletin board system (BBS) computer program, and of the genre of programs it inspired. Citadels were notable for their room-based structure (see below) and relatively heavy emphasis on messages and conversation as opposed to gaming and files. The first Citadel came online in 1980 with a single 300 baud modem; eventually many versions of the software, both clones and those descended from the original code base (but all usually called "Citadels"), became popular among BBS callers and sysops, particularly in areas such as the Pacific Northwest, Northern California and Upper Midwest of the United States, where development of the software was ongoing. Citadel BBSes were most popular in the late 1980s and early 1990s, but when the Internet became more accessible for online communication, Citadels began to decline. However, some versions of the software, from small community BBSes to large systems supporting thousands of simultaneous users, are still in use today. Citadel development has always been collaborative with a strong push to keep the source code in the public domain. This makes Citadel one of the oldest surviving FOSS projects.
The utilization of a natural metaphor, the concept of rooms devoted to topics, marked Citadel's main advancement over previous BBS packages in the area of organization. Messages are associated with rooms, to which the user moves in order to participate in discussions; similarly, a room could optionally give access to the underlying file system, permitting the organization of available files in an organic manner. Most installations permitted any user to create a room, resulting in a dynamic ebb and flow closer to true conversation than most other BBS packages achieved. Certain versions of Citadel extend the metaphor of rooms with “hallways” and/or “floors,” organizing groups of rooms according to system requirement. By contrast, previous bulletin board software emphasized the availability of files, with a single uncoupled message area that could only be read linearly, forward or backward.
Citadel further improved the user experience in the area of command and control. Based on Alan Kay’s philosophy of user-interface design, “Simple things should be simple; complex things should be possible,” and influenced by the fact that Citadel was developed in an era of 300 baud modems, the basic and most heavily used commands are accessed via single keystrokes. The most common commands are <G>oto (the next room with new messages), <N>ew messages (display the New messages in the room to the user), and <E>nter a message into the room. Other single keystroke commands exist as well, such as <K>nown rooms, which lists the rooms known to the user.