When I see an old map my instinct is to look for Fort Caroline notations.
This one says for one spot:
"Confederate Fort"
"US Fort"
"French Fort Caroline 1567" (twice)

If I read this map correctly, that would put it at the base of the bluff next to St Johns Creek.

How about a listing for the "Site of Spanish Fort" on Empire Point. This map has all kinds of easter eggs.
