Basic strategy - the tower
Four warriors arranged in a square like this are indestructible. This kind of fort (called a "tower") may be used by either side. For white, it can help by seriously disrupting black's attempts to form an enclosure. For black, it can be a last-ditch defense, when placed near a corner, if white has already seized control of vital corner squares. Of course, as soon as one of the four warriors moves, the tower becomes vulnerable to attack.