I have concluded from personal examination that they are definitely defensive fortifications. They have dugout pits at intervals just behind the walls, which were obviously positions for armed men to guard the walls and the encampment behind them. The location of the walls affords a perfect overhead view of the long lake which would be the primary route to it from the sea (the two Chainlakes).
Who built the defensive encampment? Nobody can be sure. There are no engravings on any rocks. Absolutely nothing to identify the builders. My personal opinion is that it was refugee Templars from the middle ages, probably 1400-1500. Until some identifying artifact is found, they will remain an unsolved mystery!