Thus the highest form of generalship is to balk the enemy’s plans; the next best is to prevent the junction of the enemy’s forces; the next in order is to attack the enemy’s army in the field; and the worst policy of all is to besiege walled cities.

The rule is, not to besiege walled cities if it can possibly be avoided. The preparation of mantlets, movable shelters, and various implements of war, will take up three whole months; and the piling up of mounds over against the walls will take three months more.

Sun Tsu - The art of war, 3:3-4