Update: This place has sadly been leveled by the city. I am using this spot to explain why I do not give directions to all places. Read more here.
Personal Experience: This is a fun, little spot that is tucked away in the middle of nowhere, with virtually no pathway to even find it. It is hidden amongst shrubs and trees. It would make sense that my partner found this as a child while riding his bike, going off trail. Otherwise, I don't think it would be very easy to find. So far everyone I have spoken with that is affiliated with the preserve has never heard of this spot. At this point it is a mystery. On the entrance is spray painted "Pentagram Palace" so we're going to just run with it and call it that (for now) unless we can get an official name for it! Looking at aerial photos from the 1940's, this little spot was there.
Update: It looks like this very well may be a storage facility for ammunition from WW2 which was preparation in case of a Japanese invasion. There are others like this throughout San Diego.
Member's SectionMurphy's Ranch
Anza Borrego Desert
Cuyamaca State Park