The release announcement on the www.castor.org page point to
http://dist.codehaus.org/castor/1.1.2.1/
for downloading the latest release. Unfortunately castor-1.1.2.1-doc.zip doesn't include the JDO classes nor is there a separate download. Also castor-1.1.2.1-src.zip doesn't contain the JDO source.
http://repository.codehaus.org/org/codehaus/castor/
a much better solution, as there's individual source JARs for all available modules (to be selectively selected by you, the user).