> You've evidently spent a lot of time thinking about this so let me ask you this: would there be a sweet spot that's _between_ one unit per tile a la Civ 6 and the infinite stack as in Civ 4?

Civ2 / FreeCiv is my favorite. Civ2 / FreeCiv has infinite stacks EXCEPT all 20-units die if they lose in a singular combat. (!!!) One swordman can kill 20-swordmen if the 20-swordmen are stacked in one tile. Its quite unrealistic, but it leads to very nice emergent behavior.

I do think Civ6 and Civ5's stack behavior is actually pretty good and tactical. Civ6's main problems are in the economic engine / science / culture mechanics. The tactics / war point of Civ6 is pretty good in my opinion (although Civ6 could definitely use a more complex military).

The main issue with Civ6 / Civ5 stack behavior is more pedestrian: when you have 20-units on the battlefield and none of them can step on each other, it becomes incredibly tedious and unfun to move them around. Consider a chokepoint between two unpassable mountains: you now need to click each of your 20 units to individually move across the narrow pass.

In Civ2 / FreeCiv, they just stack-up and unstack into their final positions. In Civ5/Civ6, they constantly block each other, the AI fails to reroute, and then you end up manually clicking on every unit you've ever built on every single turn until the end of the game.

In Civ2 / FreeCiv: you still don't want to stack your units if enemies are close. Put your 20-swordmen in that narrow pass, and they can die even against a singular enemy. A lot of unnecessary risk there. So this sort of movement only really happens during peacetime, far away from any risky battlefield.

--------

In theory, Civ5 / Civ6 could be fixed with some kind of AI that can perform flocking movement (like Starcraft 2's pathfinding AI: select 100 units in Starcraft, they "flock" together as a group, and traverse the terrain automagically).

Alas, this still has its flaws: flocking AIs still wouldn't necessarily put all the units you have in the correct locations. You'll have to still deal with chokepoints slowing down your army's movements, etc. etc. (Chokepoints slowing you down is tactical however: so it'd be annoying but "realistic" and maybe worth considering as a mechanic)

Still, the tedium of clicking on so many damn units so many damn times is aggravating. It really makes me wish for the old Civ2 days.

-----------

It should be noted that in Civ6, you can merge two units into a Corps for +40% damage and +40% defense. A 3rd unit can be added, forming an Army for +75% total damage and defense (once you account for +20 healing per turn with Medics, these bonuses are substantial)

So Civ6 has a "stacking mechanic" so to speak. Tactically, its fine and the 40% and 75% boosts feel rather balanced to me.

The issue is once again: economic. A 3x stacked Tank Army has the same maintenance costs as a regular 1x unit of Tank (wtf?)

So the economic engine of Civ6 is just completely out of whack. Given the high-maintenance costs of endgame units, it pretty much means you're 3x stacking everything into an army in the endgame, (which basically cuts your maintenance costs by 1/3rd). There's almost no economic cost to this strategy. Because there's no downside, there's no strategy: you just always make an army at endgame.