Citadel started in 1982. It first ran under CP/M. (requiring 64K of RAM) It was written by Cynbe Ru Taren
From the original README that came with Citadel:
Citadel is a room-structured message system. The fundamental
design goal is to provide a congenial forum conducive to interesting
discussions. The software is intended to be as unobtrusive, unintrusive
and unconstraining as possible. In software as elsewhere, good engineering
is whatever gets the job done without calling attention to itself.
The fundamental design metaphor is that of a building consisting of
a series of independent rooms, each of which hosts a discussion devoted
to a particular topic. Messages are stored and retrieved in chronological
order within each room. Messages are formatted to the caller's screen width.
Callers may travel freely between the rooms,
reading old messages and posting new ones. New rooms may be created
at will, and old ones are deleted when they empty of messages.
People familiar with other electronic message systems may wish
to compare Citadel rooms with EIES conferences, ArpaNet mailing
lists, individual "linear" BB systems or whatever; the parallels
are not exact but the functions are similar.
The fundamental Goto, Read and Enter commands have been streamlined
as much as possible. The message display format has a minimum of
unnecessary noise: the topic is implicit in the message's location
within a room, no explicit TO field is present, no message ID # is
printed, no redundant "END OF MESSAGE" blurbs etc. The most common Goto,
Read and Enter commands are all single-key. Citadel automatically
skips rooms which have no new messages, and old messages in the
current room. (Less concise commands are of course available to
Citadel Version 1 offered no more than the above, and was
quite well recieved. Version 2 leaves the basic structure unchanged,
but adds some additional peripheral capabilites. Private person-to-
person mail is now supported. Private rooms can host restricted
conferences. Once visited, private rooms behave exactly like regular
rooms to the participants, but they are not accessable to others who
don't know the name of the room. The sysop can set up some rooms to
be windows onto designated disk/userspace areas. These directory
rooms support the usual message functions, but also allow one to
to do directory listings by wildcard match, or to upload and download
files via Ward Christensen's protocol. Various rough edges have been
smoothed off. The message code has been reworked to support automatic
networking of Citadel nodes.
Citadel is written in BDS C. The distributed system can be
installed and run without recompilation in most cases. Citadel
needs CP/M 2.xx, at least 300K of disk space, an auto-answer modem,
and 64K RAM. (i.e., a 0100 -> CF00 TPA, at least). The source
files run to about 150K, the .com files to about 100K. In an
functioning system, the message and userlog files together take about
100K, and one would normally like about 200K for message text, to
keep the wraparound time longer than a week. The code is a simple
public-domain release: it can be used without fee in commercial
systems, repackaged and sold, or whatever takes your fancy. (As
a matter of good form, a pointer to the parent code would be nice,
of course.) The author takes no responsibility for the correctness,
reliability, security, use, abuse, contents or clientele of any
Citadel installation. The release of version 2.1 concludes the
author's involvement with the package.
Since Citadel was released into the public domain and came with source, it was ported heavily. It's safe to say that it can run on more machines than Linux can.
The original Citadel was taken by other people, modified, released and spread around.
First David Mitchell created some updates.
Then "Bruce King/T'an T'u" (I'm not sure if that indicates two names for one person, or two different people) made Citadel 2.10.
From Citadel 2.10 there was spawned Hue Jr.'s Citadel-86, Kerry Kyes' Citadel 2.28, Caren Park's Citadel 2.25 and Maher Maso's Citadel 2.?? (I don't know the subversion he created)
From Maher Maso's version, Babel was spawned.
From Citadel 2.28, 2.25 and Maher Maso's version was created StoneHenge, Ideatree and a bunch of New York, New Jersey and East Coast versions.
(Yes, if the internet had existed back then, the forks would have been much less likely)