We have removed Sandcastle project from Codeplex after carefully evaluating the feedback from this thread http://www.codeplex.com/Sandcastle/Thread/View.aspx?ThreadId=29085 (currently unavailable).
There are few options such as the following:
Publish the source code for Sandcastle and revive this project in Codeplex
Migrate sandcastle to MSDN Code gallery at http://code.msdn.microsoft.com
I am going to evaluate these and other options carefully but rest assured the Sandcastle downloads will be available soon for the customers.
I am greatful for all the great community efforts, support around Sandcastle and the adoption rates. I ask your patience while I make every effort to address the options presented. I will get back and blog about our decision.
Thanks for your understanding. Please feel free to contact me (firstname.lastname@example.org) directly regarding any questions. Cheers.