This term was coined by the gentlemen of the Georgia Eta chapter of Sigma Alpha Epsilon at Oglethorpe University, and is used to describe what is commonly referred to as a "fraternity house". The frat castle is the mother hub of social activities on campus.
Fratter #1: "Guys, where should we party tonight?"
Fratter #2: "The frat castle!"
All Fratters: TO THE FRAT CASTLE!!