Civilization 6, like many strategy games, has a tactical rock-paper-scissors element. Spears beat horses, horses beat swords and swords beat spears. This leads to the idea that success on the battlefield requires combined arms. Have an army made up of a mix of spears, horses and swords, where your spears are facing off against your foe’s horses, while your swords face off against their spears.
The problem with this view is that it is to small. Units do not simply exist. They have to be built. Those factors can’t be ignored just because they happened before the armies marched onto the field. As Sun Tzu said: “Victorious warriors win first and then go to war, while defeated warriors go to war first and then seek to win”.