Reportedly, he needed the bucks – he was in lots worse films at the same time.
Also, his wife, Ursula Andress, Burgess Meredith, and others were in this, too. Besides, as a big Ray Harryhausen fan, I don’t think it was a crappy movie – it was Harryhausen’s final film, and a well-done finale.
Except for that damned owl.
You want crappy?
Joan Collins in Empire of the Ants
James Earl Jones and Robby Benson in City Limits
John Carradine in Tales from the Past and The Black Sleep
I guess any actor who stays in the business long enough is going to end up in some crappy films, but some are overachievers in this regard.
I give you Jon Voight, "Star of “Coming Home,” “Deliverance,” “Midnight Cowboy” and “The Champ,” as well as An American Carol, Anaconda, The Karate Dog, Bratz, and Superbabies: Baby Geniuses 2 (currently ranked at third-worst movie ever on IMDb.)
The Madwoman of Chaillot: Katherine Hepburn, Paul Heinreid, Edith Evans, Yul Brenner, Richard Chamberlain, Donald Pleasance, Danny Kaye, Charles Boyer, and Giulietta Massina. Four Oscars, plus several other nominees on this list.