Also, you're in error about the geography. Although the fort in question was right next to Vasquez Rocks, the familiar rock formations would not have been visible in the background given the way the camera is facing in this shot. If I recall right from a Wild, Wild West episode that featured the fort and the rocks together, I believe the cliffs and such are off to camera left in that shot.
You're referring to this view, I believe:
From the POV used in the "Arena" shot I linked above, the distinctive upthrust formation would have been out-of-frame to the left, if I'm not mistaken.
Yeah, that's pretty much how I remembered it.
At the time that episode was shot, the freeway (which might have been visible from that angle, if it didn't actually pass right over the place once occupied by the fort set) hadn't been built yet, and the only roads of consequence (including the two-lane Sierra Highway, which was the only real through road) would have been behind the camera, I'm pretty sure.
I've been looking at Vasquez Rocks on Google Maps, and I think I was wrong about the location of the freeway; it would've actually been south of the park, behind the camera from that "Arena" shot. If you enter "Vasquez Rocks" on Google Maps, the "Gorn cliff" is the outcropping to the right of the "A" marker tab (or whatever you call that inverted-teardrop thing), which has a flat, sandy area just east of it. That must be the famous filming location because it's the only part that's sandy rather than covered in scrub. And there's a big rectangular "peninsula" to the sandy area on the southeast. I think that must be where the fortress stood, judging from the terrain and the camera angles. If you look behind the horses on the shot you provided, you can see a road just to the right of the big cliff, heading directly away from the camera. On the Google Maps overhead view, that road is visible going northwest from the sandy area, then curving further west and eventually intersecting with Escondido Canyon Road. That's the path my cousin and I took to get into the park; I vividly remember it because we got separated for a time and I didn't have a cell phone yet, so I trudged up and down the length of that road a couple of times trying to see if he'd gotten back to the car yet. (This was also the road that Fred and Barney drove along underneath the opening titles of the live action
The Flintstones movie, although they reversed direction on it a couple of times.)
So if the fortress was in that rectangular area in the southeast corner of the sandy region, with the gate facing northwest, it's consistent with what's in the photos. That would mean the "Arena" shot is looking pretty much directly north, assuming that's the only entrance. The only things due north of the park now are Dietz Drive and Escondido Canyon Road, and I think they'd be behind the hills from the fort's POV if they'd even been there in 1967.
EDITED TO ADD: Okay, from the Bonanza fan page
M'Sharak linked to:
http://ponderosascenery.homestead.com/vasquez12.html
The flat rocky area just behind those sparse trees and in front of the cliffs is where I think the fortress was. The camera angle is roughly the same as in the black-and-white photo through the fortress gates, but further back from the cliffs.
Having never either visited or read much about the Vasquez Rocks, I have no idea what its surroundings look like. I was simply suggesting a possible reason why the "metallic mountains" were used.
Or rather, why the crumpled metal beam that many have mistaken for a bad "mountain" effect was used.
