Born and baked in The ‘Burg, too. I remember that place! (I was 16 at the end of 1975, though. ;) ) I never got to sneak in there. That is probably a good thing, I get very claustrophobic when in underground places.
There were lots of light shafts so it never felt underground, even though we were looking at stars. But there were lots of opportunities for the first arrivals (boys) to hide around a corner so they could jump out and scare the latecomers (girls). Done by flashlight that provoked ample screaming.
We were such innocents. However, we could have flushed a rattlesnake. It was a dangerous pastime.