What did this lovely old lock secure in its past?

This was found on a wall near the Mausoleum at Minnigaff, Newton Stewart in Dumfries and Galloway? It was not obvious exactly what its original purpose was, but presumably there once had been a gate nearby. There is now so little left, if you look carefully you will see that the back of the padlock has totally corroded away.

Location: Minnigaff, Newton Stewart, Dumfries and Galloway

Photographer: Alwyn Howes