Crannog. There’s a word you don’t often hear come up in casual conversation.
So ... an atificial island. Like Venice? Or Xochimilco?
Crannog....
also Scottish slang for a boggy Colleen.
Apparently the most shallow spot would be picked, then bit by bit stones would be hauled over and dropped in until there was something that could be built on. I suspect the fortifications would have been wood palisades or something like that. After one thought of it, such things probably caught on fast, and after perhaps just a few generations the practice fell off, along with the supporting stones woich would slide down one by one.
See, I should read more:
https://www.bbc.com/news/articles/c0e2w31wedqo
An ancient artificial island in a loch on the Isle of Lewis was built entirely on a wooden platform, archaeologists have discovered.
From above, the Neolithic island - or crannog - in Loch Bhorgastail appears to be made of stone, though it was known that there was some timber underneath it.
However, researchers from the University of Southampton carried out an excavation and have revealed that wood was used to support the whole structure.
The group dated their findings between 3500 and 3300 BC - around the early Bronze Age - and said the technique would have been used at other nearby sites throughout the Outer Hebrides.