John, No trick, how did Mr Holmes and Dr Watson get there ? There used to be a railway, ran from Princetown to Plymouth via Yelverton, went quite early after the Beeching Report I believe. It is a decent walk, but it can be followed still from the moor to Plymouth. In the Austen film adaptation that has Emma Thompson, Kate Winslet and Hugh Grant you see quite a bit of it in the scenes when they travel to Devon by coach.The station was where the Visitor Centre car park is now. The sign said "Beware of Adders", I believe it is in the Shaggy Dog story, which is why I mentioned it.
My wife has reminded me that the grave yard within the prison walls has seperate sections for "French" and US prisoners of war, and then further sections for home grown prisoners following the conversion to a civilian prison. Some have grave stones or markers, especially the US ones, but many are unmarked.