Older versions of Civilizations allowed stacking units and it was possible to protect some units, for example, you could protect swordsmen from cavalry protection by locating spearmen at the same tile. As a drawback, artillery (and maybe archers?) could do collateral damage to units in the defending stacks. This allowed some interesting tactics. Later, this was removed and a tile could only allow locating a single unit. I'm curious, why this was done? To me, it was more fun to play with stacking.