Was the castle scene in Wizard of Oz filmed at Hoover Dam?

No, the castle scene in The Wizard of Oz was not filmed at Hoover Dam.

The castle scene was filmed at the MGM backlot in Culver City, California. The elaborate set was built specifically for the film.

While Hoover Dam is an impressive structure, it was not built until 1936. The Wizard of Oz was filmed in 1939, making it impossible for the film to have been shot there.