Steps to a Dark Place

I had a walk from home this afternoon to Crownhill Fort. 
Plymouth is ringed with Palmerston forts which were constructed when there was perceived to be a high risk of invasion from Napoleon's army.  The sheer scale of this fort can be appreciated from a walk around it''s outer perimeter. 
This mysterious, dark doorway is situated on the inside of the outer perimeter wall. It may constitute a back entrance or may link the main fort to the outside defences via an underground tunnel. It may be neither of these. The truth is I really don't know! The fort holds it's dark secrets well!