You can put as many units as you want in one tile, but support costs rise in proportion to how many units are in a tile. Eg. it costs more in maintenance to have 10 units in one tile that it does to have 10 units spread out. Units in the same tile can be grouped together to form Armies (and later separated if you want). An Army is a group of units that moves and fights together and costs even more to maintain than if it were just 10 units in the same tile. Good Armies are much more effective that the same number of solo units. Eg, if you attack with an army of 10 Spearmen it's more powerful than 10 Spearmen attacking one by one. However, a diverse army with 3 Spearmen, 3 Archers and 3 Chariots would be stronger still. When Armies fight and lose sometimes units within are destroyed, sometimes damaged depending on the units used by the winnner and the losing Army's composition. Veteran units can increase your chances of killing the enemy or surviving themselves. Great Generals are generated through Army success and can lead Armies to make them more efficient & powerful. The more units on one tile, the more effective using artillery against that tile becomes. A Catapult that attacks a lone Spearman would do some damage to the Spearman, but if it attacked a tile with an Army of 10 Spearman it would have a chance to damage several units.