But wait, just outside the fort's gate there's one final encroachment by the lumber industry! Maybe. From the sign:

These ruins, built in the 1800's are not associated with the fort.
They were built from rock, cobblestone and sea coral brought in as ballast on the large timber schooners.
The ruins
(sic) purpose is unknown although test pits dug here revealed burned wood and other material,
indicating the structure may have been an incinerator for sawmill scrap.