A desert mystery written up by Patty Furbish in her first edition of On Foot in Joshua Tree National Monument, 1986 (since reprinted several times, updated to *On Foot in Joshua Tree National Park*, and a near essential book for the avid hiker and explorer of JTNP)
The book gives a major clue, siting *the official location as Section 13, T5S, R12E.* Many people have read this, whipped out their topo maps, and narrowed the location down to a square on the map on the Park's southern border. Upon following a wash from that border to a springs located on the map, and poking around a bit, this *castle* pictured above is apparently not all that hard to *discover*.
I've never actually been there myself. Dan Martin, frequent poster at The Joshua Tree National Park Message Board, has this to say about the hike. *it's a long, tedious slog with some scrambling and roughly 1300 feet of elevation gain through several different canyons. It's not an easy hike, but it's not technical either.*
Dan also provided the above photo, by way of posting it on the message board.
Here's an online slide show