I heard it was center of the main room, so those old pictures you see of people sitting around it was actually the main room, before it had a roof.
The terrace didn't really exist back then (late 80's) as the walls and roof were built up around it later on, I remember when I first went the terrace was a big greenhouse and it had low level music, so was more of a chillout area, the main room was where it all happened.
In the 80's it was a 1 room open air club from what I was heard, would loved to have gone!