Current release: 4.1.0
CTWM has historically supported (to some extent at least) a gigantic list of platforms. But most of them are now defunct, and have been for many years. As of 4.0.0, we're no longer worrying much about breaking older systems. If you're using CTWM on something significantly different from a reasonably modern *nix system, there's a fair chance it'll break at some point.
We require a compiler with at least basic C99-level capabilities, moderately current POSIX compatibility, and some non-standard but common extensions like getopt_long(3) and asprintf(3).
We'd be happy to hear from you on the mailing list if things do break for you, and if you're willing to help test and shepherd the platform longer term, we're happy to work with you to keep it going. But platforms out of the ordinary that lack an active voice in the community are probably going to be left behind. Sorry.
CTWM is currently managed via git with the primary repository on github.
The main repository is at https://github.com/ctwm-wm/ctwm.
This is historical information, not current.
The CTWM source was previously managed using Breezy. The repository was hosted on launchpad, with the main branch being lp:ctwm See the project page on launchpad for more. Formerly Bazaar was the recommended program for interacting with the repository; it would still work if you have it, but since it requires python 2.7 it's no longer recommended (or even easy to get working on many platforms).
This is historical information, not current.
Here is a crash course describing how to use brz for developing CTWM.
This is historical information, not current.
A semi-official git mirror is maintained on github at https://github.com/fullermd/ctwm-mirror. Note that this is a read-only mirror, so using it as a basis for development you intend to contribute back, sending pull requests, etc, won't work any better than just sending patches. It's mostly useful to fetch the source for people more comfortable with git, or for special situations where using brz isn't an option.