Spiral might like this... the tip of north-east Scotland is called 'The Knuckle'. The tip, Rattray Head, is marked by a lighthouse, connected by a causeway that's only accessible at low tide.
It's hard to tell from the sparse description whether the causeway was built in the nineteenth century by the Stevensons and their team or whether it was a pre-existing feature. Rattray however was the site of a castle built on 'a rock near the sea' which might fit the bill.
Rattray became a royal burgh in 1563-1564 and at one time had its own castle on ‘Castlehill’ built by the Comyns. It is thought that this castle which once stood on a rock near the sea was burned to the ground by Edward Bruce younger brother of Robert the Bruce during the ‘Harrying of Buchan’, in 1308 or another less credible version is that it was overcome with the shifting sands. Rattray once had a good natural harbour but by 1654 it was becoming badly silted.
Before the entrance was silted up, this part of the coast had an opening into a 'land-locked salt-water basin'. Looks or sounds vaguely familiar. This sandy coastline, measured as 25,000 yards, is known for its nine castles, the 'Nine Castles of the Knuckle' so presumably most or all earlier constructions, if any existed, would have been subsumed into later projects.
Rattray, Rattery, Rathen...variations of 'Rat' names, something we puzzled over briefly in connection with one or two Rat Islands.