Running: 2x speed. Automatic, not triggered by player. Usually when catching up to formation, when "kiting", or when running away from attacker (female citizens) Charging: 2.5x speed (infantry), 3x speed (cavalry). Manual, triggered by player. Units will not start the charge until they walk to within charge distance. Units will not charge if too close to target. Stamina used simply as a cool down for the ability (must rebuild stamina). Stamina bar shows in UI but not in unit status bar. Some units (cavalry) have more stamina than others (heavy infantry), so can charge 2 or 3 times before cool down required. Charge give 2x attack (infantry) or 3x attack (cavalry, elephants, chariots) on contact. But charging units also receive 1.5x damage from retaliating target units on first strike. Very simple. Some thoughts for: Make spear cavalry have a heavier charge than sword cavalry. Viceversa, make sword cavalry have heavier standard attack than spear cavalry. Same goes for spear infantry and sword infantry.Battering Rams can "charge" as well, against building.Cool down/stamina, charge bonus, etc. open opportunity for technologies, e.g. "Hoplomachia" - Increased running and charging speed for Hoplites. "Othismos" - Increased charge attack bonus, "Shield Bash", "Hippika Gymnasia" - Increase stamina for cavalry, etc.