well all i know was from car park over fence downa very nasty path to this there where other small looking entrances that where filled..but main tunnel was about 80 yeards long was just glad to get too carpark level me legs gave out and it was clamber on all fours up a very steep slope..stick me down a manhole deep as u like cliff edges nah..may be worth a look again ,some local bloke in car park mentioned it as he saw rucksacks etc we hadI've Got no idea - it looks just like the passages inside part of the detached bastion in Dover - and while St martins isn't all that far from there, I don't think that's where you were. It's clearly part of the napoleonic defences thou.