How to get to Degmar, The Lost Castle
In order to zone into Castle Degmar you'll first need to complete all of the TDS progression up to and including Caverns of Endless Song. After you do that you'll be flagged for access to Castle Degmar. Once you're ready to enter Castle Degmar you'll want to travel on over to Brother Island first.
You'll find a blackened statue (pictured below) in the north-central part of Brother Island, clicking on it will zone you into Castle Degmar.