Arguably the most popular of the MUD codebases, if not at least the most popular one by Implementors/Admins from the Diku family.
Many were the coder and admin who eagerly awaited every single release from Circle, year after year. Every single patch was quickly downloaded and applied to existing MUDs, fixing bugs, increasing stability, and advancing extensibility.
CircleMUD also enjoyed help from the Diku community at large, constantly provided with new areas and "snippets" of functionality (e.g. spells, classes, mobscripts, etc.) to either be incorporated into the official releases, or as something to be shared among admins to freely add to their own MUDs.
Circle is considered to be one of the most highly advanced and evolved MUDs, providing a staggering amount of ready-to-play areas, skills, spells, the four basic classes, Mob scripts, and documentation. That documentation was not only impressive, but the extensibility of the project made a perfect match. It was the simplest thing for a coder to add in a new class, a new spell, or a new "Smart" mobile in a single sitting.
A lot that Circle has to offer comes as a result of the amazing work already pioneered by DikuMUD. While its contemporaries, such as Copper and Merc were also impressive, Circle took the "hack-and-slash" MUD coding world by storm.There was a particularly long period where development paused between the 2.x and 3.x tree. However, there was quite a bit of good material and updates from the 3.x series.
Eventually development stopped completely with the extremely mature CircleMUD version 3.5. The project website is still operational and contains much more information at CircleMUD
The successor to CircleMUD is considered to be tbaMUD, picking up directly from version 3.5 of the Circle code.
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There once was a guy who played MUDs
But the MUDs that he played were all duds
“I’ll write one,” said he.
And he brushed off his C.
And soon he played his game with his buds.
-- Background Story, CircleMUD