In honor of Mary Stewart, I re-read "The Crystal Cave." I'm currently somewhere in the middle of "The Hollow Hills."
Mary Stewart uses deus ex machina at the end of "The Crystal Cave" well. If you look at the Geoffrey of Mallory version of the Arthur stories, they're pretty messed up, but Stewart manages to salvage the events around Arthur's conception: With the aid of Merlin, Uther and Igraine are able to conceive Arthur at the expense of Goloris because... God wills it. It's not an extra-marital cuckolding on the slimmest of technicalities, and Stewart writes Uther's wrath at Merlin over the arbitrariness of the matter so convincingly, that she pulls the reader's attention away from God pulling everyone's strings.