This is the proper way to refer to the place where all your tiny square shaped burgers are made. Those in the St. Louis area (but also some parts of Detroit) refer to the beloved fast food chain with a plural ending (even though the company name is singular). Thoughts are that the term originated in areas like North County where the restuarants are so prevalent that you can say I'm going to 'White Castles' and literally mean multiple stores because there are ones on every corner. Some refute the plural endings, but those belly bomber lovers out there know the truth.
I'm finna drive till I find a White Castles and get two double cheese and a coke and orange mix.