Visited July 2009.
For directions see Martin’s post below.
I stopped off here on the way back from a visit to Edinburgh. There is a T/A building next to where you park and signs stating that you can’t visit when the flags are flying – which means there is live firing taking place! Luckily when I visited all was peaceful and I had no trouble entering the Souterrain through the unlocked metal gate. The site reminded me very much of Fogues I visited in Cornwall this year – very similar – so much so that my guess is they were used for the same purpose. Well worth a visit if you happen to be in the area.