That movie "The Keep" was one of my faves in the sci-fi/horror genre. It doesn't stand the test of time very well (largely because of the heavily synthesized 80's musical score by Tangerine Dream and the 80's direction style of Michael Mann), but it's still fun for me to watch and remenisce.
A much younger Ian McKellan, Scott Glenn, and one of my cult fave actors, Jurgen Prochnow make for a well-acted movie, in spite of its 80's-ishness. The concept is so cool. A demon imprisoned by a talisman in an old stone keep stalks Nazis who occupy the keep, gaining the appreciation of the Romanian locals. The demon's assault on the Nazis tricks the locals into believing that releasing him from his prison will turn the war in their favor. Such a novel concept. That's actually one I'd like to see remade some day.