In CiV units can be upgraded as time goes by. For example, a Warrior can become a Swordsman, a Longswordsman, a Rifleman, an Infantry and finally a Mechanized Infantry. This is particularly important because units get experience so after fighting many battles your Warrior can upgrade and retain all of his combat benefits from experience. If you play well you can end up making only a handful of units in the stone age and simply upgrade them all right into the modern day. Unfortunately for the base game these upgrade paths applied to only a few specific units and others simply became obsolete. This means of course that in a long game any unit without a proper upgrade path is going to be junk for the player, but fine for the AI since they rarely manage to keep their units around for long timespans.
The first solution to this strange imbalance I saw was by Thalassicus. Thal added some new upgrade paths to the game, though they weren't quite as straightforward as the one in the paragraph above.
Spearman to Pikeman
"So Thog, we used to use spears, now we are supposed to use these pikes instead? Is this gonna work?"
"Yes Grog, just point the sharp end at the guy on the horse, same principle."
That works. The next step is a little stranger.
Pikeman to Lancer
"So Thog, we used to stick people on horses with pikes, now we are supposed to put the pikes away, get on horses ourselves and learn to use lances?"
"Yes Grog, we used to smash horse units, now we just do it from horseback."
The next step is very strange indeed.
Lancer to Anti Tank Gun
"Thog, this is BS. We just learned to ride horses and now we have to get back off our horses and use these crazy tank guns? Why did we go to horseback back there?"
"Grog, just point the gun at the tank and stop trying to think. This is a game, not a simulation."
The last step:
Anti Tank Gun to Helicopter
"Thog, I fought horsemen for 3000 years and now you want me to learn to fly a damn whirlygig? I quit."
"Wheeee, helicopters are fun!"
So Thog and Grog the spearmen end up piloting helicopters. This is the best path available, despite how bizarre it looks, so I basically accepted that all units should be carefully levelled up and survive to the modern era with incredible skills and powers. The problem with all this is that in the game you really do only make 10 military units, all of them in the first 1/3 of the game, and by the end you have an invincible army of supermen. The only way the computer can deal with this is to throw more and more dudes at you, but that just levels up your units even faster, to the point that sometimes in the endgame you are zerged by so many units that moving foward is impossible; they fill every space with a dude. This is a terrible situation.
The solution I came up with today is simple, and basically the opposite of everything else that has been tried. I removed all upgrades completely. Thog and Grog the spearmen may be level 10 and have every ability in the game but we are in the medieval period now and they cannot stand up to heavily armored opponents. They must retire (disband) and the player will actually need to make more dudes. It should feel like military spending is part of the game again because an army will need a constant supply of new, upgraded troops. Also because the player isn't going to have ludicrously high level units all the time they are likely going to have units die fairly regularly. I suspect that this change will make the highest difficulty levels pretty much impossible but I think it will make the game feel a lot more natural and make the AI much more challenging without having to resort to giving it such stupendous cheating benefits.
Lots of people won't like my solution very much because it is fun to have an incredibly powerful army of units who have been around forever, but to make that game hard requires giving the computer far too many silly bonuses. I want a really tough challenge that doesn't involve dozens of hours of wading through endless hordes of enemies; just because the game is hard is no good reason for it to take 4 times as long to click through.