http://blogs.msdn.com/codeplex/archive/2008/06/06/removal-of-sandcastle-project.aspx

Looks like Sandcastle is gone… hopefully not for long.
M$ feel it is not appropriate to have SC on CodePlex as its source code is not up and CodePlex is an OSS site/repository…. so fair enough, hopefully they find another more suitable place for it and dont keep it a secret.

NB: There are some links for the last release on the link above.