This mechanism elegantly portrays the gradual disorganization and loss of cohesion an army experiences during combat. Good differentiation between the various unit types. Heavy infantry are powerful but difficult to maneuver, as they were historically. Only the top unit in a stack may attack. Some of the Egyptian phalanxes were inexperienced, and their morale level is determined only when they are first involved in combat.
Leaders give a morale boost to any combat units they are stacked with.
Having your leader killed in combat or routed off the map incurs a significant negative modifier to any subsequent morale checks made by any unit in your army. The rules state that no unit may be forced to defend more than once in a turn, and that a group of more than one stack may not combine to attack other units. While this may be true, I still think that the rule is too restrictive. The rules would seem to prohibit that strategy. It seems to me that attacks through a rear hexside should be made against the bottom unit in a stack.