They tried to hold back the sea.
That’s one of the interesting possibilities. The object’s age, if correct, correlates somewhat with rising sea levels due to “global warming” at the start of the present Interglacial. But let’s learn more about the rocks first.
I suspect they were trying to keep out human enemies. It is much easier to move inland than to build a structure like this.
Agreed. Looks like rip rap to me.