Salt Lake is an Outer Domain with the Main Building producing special resource – the Black Gems. The Lakes have limited lifespan; you can receive information on that inside the Main Building or right on the Main Building.
There is neither Gate nor Tunnel in Salt Lakes so it can be attacked anytime.
There are 4 cells on the hovering island that you can use for constructing defensive or other buildings. Salt Lakes also have villages where buildings can be constructed.