At https://phys.org/news/2019-06-evidence-scottish-crannogs-thousands.html … a research article in the upmarket archaeological journal Antiquity (June 2019) DOI:10.15184/aqy2019.41 … archaeologists with the University of Reading and the University of Southampton (the latter specialises in under water locations) claim there is evidence crannogs (which occur in Scotland, Wales and Ireland), small artificial islands located in lakes and in areas prone to flooding, were actually first built during the Neolithic period (in the 4th millennium BC). They were recoccupied in the Iron Age – which is why archaeologists have assumed they date from the first millennium BC. There is a possibility the water table was much higher in the early to mid Iron Age (in the UK) which receded during the Roman Warm Period (after 200BC) only to rise again in the 5th/6th centuries AD. Fascinating subject. Occupation in the Iron Age moved from river valleys to higher ground and the same thing happened in the Late/ Post-Roman period. The Romano British built their villa estates in valleys and close to rivers as the flood plain provided first class agricultural land.
Most crannogs are no bigger than 10 to 30m in diameter and appear to be little more than family units. The water provided protection but nobody really knows why they were built and what the threat to the occupants might have been. It could have been defensive, for example, or an attempt to adapt to rising water levels.