Stacking military units hasn't gone back to the same system as 4, thankfully. It's more a means of decluttering the map in the late game when you're able to produce a lot of military units, as you can now combine them to upgrade them, and attach support units, rather than have them take up a separate tile.
http://www.polygon.com/features/2016/5/ ... te-previewUNIT STACKING IS BACK
As mentioned before, in Civ 5, military units can generally only inhabit one space. This is only not true when you have land, sea and air units in the same space, or when you have multiple air units.
Now, players can attach very specific land units to one another in order to create symbiotic boosts or to create limited "armies" made up of the same unit. We saw this used in Civilization Revolution — the console version of Civilization — in which it was barely worth fighting until you had created a multiple unit army.
"WE HAD TOO MUCH CONGESTION ON THE MAP."
"When we unstacked the armies in Civ 5, all the tactical nuances of having cavalry and archers and melee units separated on separate tiles created little rock-paper-scissors combinations that were very clear for players to understand," says Beach. "That was a beautiful win.
"The problem was we had too much congestion on the map. All those spread-out units took up too much real estate. We wanted to think about clever ways to combine the units, which didn’t lose that tactical mini-game. We found that there were two or three areas we could combine units together and it didn’t hurt the tactical nature of the game.
"The first was that there were a lot of units that were just additional equipment for your units. It might be a battering ram or a siege tower or an anti-aircraft gun or an anti-tank gun. In Civ 5 those were all special units that took up a whole dedicated tile. In Civ 6, we call them support units. They can stack with other military units without you having to worry about managing them on a tile by themselves.
YOU CAN ONLY COMBINE UNITS OF THE SAME TYPE.
"The other part was, we found that once you got your production going in the middle to late part of the game, sometimes you could have lots of units of a certain type. We felt like it was a more realistic part of military history at that point in time if you could concentrate your forces better and achieve what we called corps or armies, where you take two or three units and stack them together.
"That unit is going to have a lot more punch, probably have the ability to drive a hole right through the enemy battle line. That’s how that system works. Now, once you link two units into a corps, they’re individual units, but at that point they’ve been upgraded. You want to keep them in that fashion if possible.
"But you always have to combine units of the same type. You have to take two riflemen and put them together into a riflemen corps. Or you have to take three tank units and create a tank army with them."