Is it necessary to give players a "landmark" when defining where to set the defensive line? I'm particularly curious about areas close to, or with in shooting range in the defending third. Do you tell the players to allow the goalie to cover anything inside the 6, etc? Are you defending with offside in mind, or focused on being in position to clear the ball? I have found that getting an offside call at the High School level inside the 18 is near impossible, and very frustrating.
I concentrated on placement of the line of confrontation, pressure on the ball and balance from the first line of 4, and then cover and balance from the second line of 4. At a certain point the zone becomes man to man marking. Also at a certain point the keeper provides cover behind the back line instead of the backs. When and where these changes happen depends on the ability of the players. For instance some keepers can sweep the area behind the back line 30 yards from the goal line. Others can't. Yes, I always want players to keep the offside law in mind. Otherwise the team marks opponents that don't need to be marked. This is important close to the goal line. Keepers don't man mark. They stay ready to go win the ball, and they provide cover. I would suggest that defenders defend with either "marking an opponent" or "defending an area of the field" in mind. When they assume the 1st defender role, I want them to focus on either winning the ball or forcing the attacker to play the ball negatively. Either win the ball or prevent the attacker from turning to goal. Clearing the ball is never plan A, but sometimes it is the best the defender can do. Just like sometimes the back gets beat and the best he can do is try to recover and (shoulder) charge the attacker away from goal while he is shooting.
Below is my understanding: when the ball is played in the middle, defenders' landmark should be the top of the box i think. when the ball is played in the wing deep near the corner, center backs need to retreat into their box to defend the cross.
Pardon me from saying so, but this advice is flawed. What happens when opponents don't play in the narrow way expected? Who marks the attackers inside the box if the CBs "defend the cross"? What if the opponents don't have wings or the wings don't run down near the corner before crossing. What if they turn inside or cross early? I have also played teams whose players could not keep a cross on the field of play, much less into the box. I think the better practice is that every opponent inside the "danger area" needs to be closely marked. There are special defensive organizations for restarts, but, especially for high school, it is too confusing to change defensive systems during a defensive moment of the game. Keep in mind though that every zone becomes a man to man coverage in front of the goal. It is simply too dangerous to leave opponents unmarked in front of the goal. As a coach I didn't tell players that we changed our coverage to man to man in front of the goal based on a landmark. It is just common sense not to leave an opponent unmarked in a dangerous position in front of the goal. Eight year old novices understand this. I just reinforced common sense decisions about defending in the "danger area". The danger area is a fluid concept, not a fixed landmark on the field. It varies in size and shape based on the ability of the players. Another problem I have is that if you objectively define the danger area to U10s by landmarks on the field, then you are teaching players how to play against U10s instead of how to play soccer. In development I taught general principles and concepts that the players could use in the future too. This allows the players to develop judgment and decision making instead of following instructions. It also results in a stronger team and better play.