Assume you had a genius AI, how would you solve the relative sizes of tiles vs armies vs world, the production times that these tile sizes imply in order to keep army sizes small (and the unbalanced relationship this creates between large and small city outputs in all categories), and the traffic jam nature of combat in a battlefield represented by tiles this large in relation to the rest of the world?
A fair question.
I honestly have no problem with the relative sizes of armies vs the world.
I have no problem fighting my way up an Italian peninsula that is 5 tiles across, or shooting across a lake. Its a game, with abstractions to get flavor. So I think this is a non-issue.
Why should this size abstraction be worse than any other in the game?
Its not like there are really only 4 cities in France, or that all population lives in cities, or that it takes a year to cross an ocean, or that wars can last a century, or any of the other abstractions that we're just fine with normally.
I think the solution with large/small city combinations is basically to increase the production capability of larger cities relative to small cities, while increasing production costs for units at higher techs (and upgrade costs), while ensuring that higher tech units massively dominate lower tech units, which Civ5 already does a better job of than any previous Civ.
This is achieved somewhat in some of Thaliscus's Balance mods, which basically doubles the bonus of the workshop, forge, and windmill buildings.
I don't have a problem with a "traffic jam" nature of battlefield in the sense of congestion making a meaningful impact. It should be harder to advance an army through a narrow gap, you should be able to use terrain chokes so a small army can resist a large one. Not every
I think playing Battle for Wesnoth gives a great feel of how restrictive spacing can matter.
If you have a massive army, don't try pushing it all through a choke, go use your navy to open another front somewhere else.
I would say though that with a genius-enough tactical AI, then we can tone down the happiness bonuses and unit production bonuses at the high end AIs while remaining competitive, so they won't need to outnumber the human 3:1 to have any kind of chance, so you won't see a sea of enemies so much.
It all rests on the tactical AI though. Without that, its not really going to work.
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Also, I really object to Sulla's "you're a fool if you think that Civ4 combat is just building a big stack and throwing it at the enemy".
Guess what, for most players (and the AI), that is exactly what they did.
And the reason it worked ok was that the advantage from doing anything more sophisticated than that was very small relative to the advantage from doing exactly that.
For the vast majority of players, combat in Civ4 was really boring.