The Fictional Location

The opening Credits are shown over a background of a map showing a castle like building in the upper right corner. In the middle top is a feature which is labeled "Death Valley".

If the map is oriented with north at the top, the castle like building would be east of Death Valley and probably in Nevada.

Later in the movie the Manderley Castle is said to be 35 miles from Mohave Wells, California. Thus the Manderley Castle could be no farther than 35 miles into Arizona or Nevada and can very easily be in California.

There is a town of Mojave in Kern county, California, but apparently no town of Mohave Wells. Mojave is about 50 miles east of Bakersfield, and southwest of Death valley.

The most famous, but not the only, "castle" in the Mohave Desert is Death Valley Scotty's Castle, in Death Valley, which may have been an inspiration for the Manderley Castle.

https://en.wikipedia.org/wiki/Scotty%27s_Castle

Another castle in the Mohave Desert is Shea's Castle, just west of Lancaster, California.

https://en.wikipedia.org/wiki/Shea%27s_Castle