The first thing that springs to mind when I think of William Castle is House on Haunted Hill, with it's skeleton being brought out into the audience on strings. And then there was The Tingler with it's electrified cinema seats.
So yes, I guess the gimmicks are kind of part of his legacy, but the films themselves are fun and, like Ed Wood, I don't think he ever made a film that could be accused of being boring.
He may or may not have made a classic, but to have left a load of boring films behind would be a worse legacy, I suspect. Thankfully he hasn't.