The main set of the film was built in a disused abandoned former slate quarry at Glyn Rhonwy near Llanberis in North Wales. Some interiors of "The Keep" were filmed inside the natural stonework of the Llechwedd Slate Caverns near the historic mining town of Blaenau Ffestiniog in Gwynedd, Wales. Michael Mann once described the set by saying: "It's a black monumental structure that might have been built by a medieval Albert Speer."
The writer of the original novel, F. Paul Wilson, was so unimpressed by Michael Mann's adaptation of his work, that he wrote a short story called "Cuts", in which a writer puts a voodoo curse on a director, who has mangled his work.
Michael Mann once described this film as: "A fairy story for grown-ups. Fairy tales have the power of dreams, from the outside. I decided to stylize the art direction and photography extensively, but use realistic characterization and dialogue."