Let's cut to the point: What if you could stack units in cities, but you need to build special buildings to do so? For example: 1 Barrack could host 1 Unit. So you could build multiple barracks, raising the capacity +1 for each. This is also kind of realistic, as you need a place/building where your troops can stay in real life too. The same could work for Airports/Airfields. This would also enhance the rest of the Civ V gameplay greatly in return: Empires would spread out more, as there is an important reason for building cities etc. This also would not lead to defensive Stacks of Doom, as you can only stack as many units as you have buildings/space for them, and this buildings can be expensive and take a while to build. So no too big stacks, no overpowering. I had this idea when I thought about the 1UPT rule. I really like it and it's good, but it just feels wrong when it comes to the cities. A big mega city can only host 1 Unit? Hmm but with "Unit per Building" things would just make sense, I really think this is something the gameplay really needs.