If events are keyed to year dates rather than turns, then the main obstacle to using six-month turns is that it puts the 12th century renaissance on steroids.
I've only started playing the game, but I'm assuming that if you play the game at the nomal pace, by the time you get to the fifteenth century you'll have a little light artillery, a few handgunners, a heavy cavalry contingent decked out in heavy plate armor, a few infantry units decked out in similar fashion, and a variety of things that just wouldn't have appeared on a battlefield during the time of the Crusades. You would also have a bunch of social and architectural structures that you wouldn't find in an earlier period, like big post-gothic cathedrals and a network of guildhalls.
If you put the 12th century renaissance on steroids by playing at one quarter speed, you'll probably get all that and the infrastructure that goes with it well before the mongol invasion.
Now I realize that most people's idea of what the Middle Ages looked like is the cavalcade room of armor in New York's Metropolitan Museum of Art. But it just ain't so. What you're looking at in that room is really the start of the sixteenth century, and the reason the non-jousting armor is so thick is that they needed something that, among other things, could protect them from primitive firearms. This gets back to my complaint about generals running around in early 16th century armor commanding troops dressed in 12th century-compatible equipment -- fine in the 16th but downright weird looking in the 12th.
[I'm not going to discuss architecture because there's not much that can be done about it. Except at the very end of the time-frame covered by the game, MTW and RTW had more correct walls -- though I haven't seen Byzantine or Middle Eastern fortifications yet].
To avoid fielding ahistorical armies in the 12th century (which is what I'm really talking about) without doing a major overhaul of game mechanics (consider that though the EB team is doing a truly epic and admirable job, they're only up to 0.81 and are probably years away from 2.0 -- tinkering with the guts of a game causes CTD's and other problems) I think what you have to do is slow down the rate that spare cash accumulates. One way to do that is to simply make the cost of everything except upkeep four times as expensive, though even that formula is probably too generous. And a side effect would be that the game would probably get weird on you because the AI would go bankrupt faster than you would. This could lead to a situation where you were the only faction with money, armored troops, and stone buildings, and everyone else would be living in dirt hovels (hey wait a minute, that's what the Middle Ages was actually like...).
I don't know how or where to do a global price increase, or even if it is advisable. But I'm reminded a bit of Asimov's Foundation Trilogy, where the most perfect sociological evolution model gets out of whack and stops reflecting reality after a few years. M2TW is just a game, but it is also a model, but a model that doesn't try to mimic history too closely. It's a pity that there is only one Grand Campaign, rather than three (or perhaps four) for different time periods the way it was done in MTW. I also miss the MTW alternate victory conditions. Conquest of Europe in the Middle Ages by anyone but the Mongols was unthinkable, if for no other reason than the factions didn't even have the resources to control what we would think of as their own countries, let alone anyone else's.
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