The best defensive strategy is to win the ball as near to the opponents' goal as possible. This can happen if the team retains a compact shape. A compact formation makes three important things possible for a defending team. It means the player on the ball can be challenged quickly. This prevents him, or at least delays him, from playing the ball forward.
Players are available to track opponents making forward runs.
The chances of conceding a goal will be reduced further if players defending in and around their own penalty area can achieve the following four objectives. Being first to the ball. Clearing with height, distance and width. Defending the area of the far post. Not getting caught in possession in the defending third of the field.