Can you recall how long you sailed from Calabria before you arrived at this spot for that would help limit the suggestions I could make. We've cruised the area so may be able to help.
Mallorca (and capital Palma) is a good 260 miles due West from Sardinia, maybe 1.5 or 2 days west - frequented by lots of Italian yachts in summer months - but not really on a direct route to Gibraltar.
I also know my own memory can be suspect - but if you can recall more detail it may help us with further suggestions.
It may even be one of the islands off the west coast on Sardinia?
There is no high land around the bays on the north side of Formentera so sadly I think you can scrub that previous suggested location off the list.
However, there are quite a few small and larger islets off Mallorca and Ibiza - and some have very enclosed anchorages which might appear to be vocanic when one is rubbing ones eyes in the early morn.
Indeed, even the port of Soller on the NW coast of Mallorca is situated in an almost perfectly round bay - and it does have an abandoned Spanish Naval Base / almost castle at it's entrance - but more info might disqualify it.
EG Can you recall approximately how high the surrounding countryside reached? Was it vegetated? If not - what colours were the soil or rocks? Where did you move onto next - and how long did it take to get there?
I guessing that a 30 foot wooden yacht might average 5.5 knots - meaning you'd cover 120 - 140 miles in any 24 hour period. If you can recall averaged speeds - that also could begin to steer us in the right direction
Looking forward to hearing if you can recall more.