4.Strong Squares/Outposts

This is one of the most recurring themes in chess - outposts and/or strong squares. An outpost is a square that can be controlled or protected by a pawn but it can't be attacked by an opponent's pawn. The terms outposts and strong squares are often intertwined, where the main difference would be that strong squares may not necessarily mean that it's impossible for the opponent to control the given square by a pawn, yet that it is difficult to do so.