All the so-called "dips" were gun emplacements and associated underground storage and chambers. When I was at St Georges in the '50s they were weakly fenced off with wire and very accesible: also very smelly.
I think it was probably around 1953 that I used to play in the concrete bases of the HAA gun site at New Brighton (that's all that was left then)& it was situated in what is now part of the miniature golf course, before & South of the clown roundabout. In those days there was nothing else built on the so called "dips" at all. I was also at St. Georges.