I remember the days when you could walk along the roof of the piers at Gatwick.
Lots of trans-Atlantic charters and other interesting stuff.
TIA, Capitol, World, Saturn, Pacific Western, Canadian Pacific, RCAF, etc.
I was just thinking that I remembered walking a good distance along the roof of one or more of the piers at Gatwick, you really felt like you were in among the apron's goings on. When did that stop ? Certainly by the 90's.
Has there ever in the history of aviation been a terrorist attack carried out from a viewing area? I could be wrong but I suspect not.
I'll second El Bunto, but I do remember the great spotter's area at Paris Orly in the 70's that had robust netting over it's 'roof' and when I asked the two machine gun toting CRS officers walking up and down it's length why (in sign language ;-) ) they indicated that it was to stop grenades being thrown over to the very close aircraft and fuel trucks. This might have been in the post Arab/Palestine hijacking/terrorist period.
This reduced area through the 80's and 90's was accessed through what I think was supposed to be a fire exit door in the side of Terminal 2 before you went up many flights of stairs to access what was left. There was an entry ticket machine but don't ask me how much it was then as I can't remember.
That's how I remember my last visit. Entry was around £2ish.