There are two backup strategies you can use, depending on your site's availability requirements and disk space availability. This section of the system administration manual goes more into deep.
The first is, to copy the entire citadel/ directory. The most important files are citadel.config, citadel.control, and the data/ subdirectory, but there are other files which contain non-static data as well.
Since Citadel uses a journalling database backend, it is vitally important that you backup both the data files and the logs, and that the data files (data/cdb.* ) are always backed up before the logs (data/log.* ). The logs must always be backed up after the data files. It might be apropriate to move the logfiles to another location.You can however speed this up by using LVM and Snapshots
The second, slower one, is ARTV Export & Import, that dumps out your database into an ascii stream.