Their bonus is the most effective against heavy units, but weak against fast units and light units when standing in forests. For this reason having some light units present helps negate that threat.
Try to avoid to artillery units, so they can be avoided in 2 hits vs opponent artillery units in all cases at all same age. Generally heavy units, light units and late game fast units can do the same. Having low defense does makes extremely vulnerable to all sources of damage without receiving counter attack and not highest speed, but only their passive defense bonus vs heavy units, they rely heavily on units to protect them. Fast units are being very fast and able to charge to any past their short range without being attacked. They receive an attack bonus against heavy units, which can be very effectively eliminate opposing heavy units walls and take less damage from heavy units, but are not guaranteed to kill enemy heavy units in 2 hits at all same age. Up until the Progressive Era, ranged units receive attack bonus from rocks, they are usually one of the second unlocked units in each age and have the very high attack, but low defense, moderate speed and short attack range in all same-age units. Ranged units ( ) are military units class with short attack range.
