Steppe Merc

, here's an example of what I've done...
You probably already know which file to edit, but just to rephrase it and make it clear to others, it is the desc_battle_map_movement_modifiers.txt in the /data directory of your R:TW game.
These are the default movement settings for terrain as released by CA.. Note how default movement is pretty much the same for all terrain except forest, ice and snow. These values affect movement rates for all units. (The movement modifiers for individual units/types must be in some other file, or perhaps hardcoded, as has been suggested)
Note the 1.0 for grass_short, etc. - this is the default highest unit rate.
climate default
grass_short 1.0
grass_long 1.0
sand 1.0
rock 1.0
forest_dense 0.75
scrub_dense 1.0
swamp 1.0
mud 1.0
mud_road 1.0
stone_road 1.0
water 0.6
ice 0.5
snow 0.75
My settings are below (This is an example of the faster version of my mod - initially I had cut movement rates in half, then I decided to raise them slightly, since towards the end of battles exhausted units would move akwardly - creating a "treadmill effect", as someone else already mentioned)
climate default
grass_short 0.792
grass_long 0.792
sand 0.792
rock 0.792
forest_dense 0.6
scrub_dense 0.792
swamp 0.792
mud 0.792
mud_road 0.792
stone_road 0.792
water 0.48
ice 0.396
snow 0.5
Anyway, that's all you need to do.. Play with the settings until you find something that suits you. Since movement rates is only part of the problem (for me and some others at least, not the people that think the movement and killing rates are "just fine" and that we should all get used to it); I think I should mention that Adonys at twcenter is working on a mod to decrease the killing rate of units, I've tried it already in a few battles and it's coming along nicely. Here's a link in case you didn't notice Sid Quibley's post about this in the Entrance Hall:
http://www.twcenter.net/forums/index.php?showtopic=9877
.....
Capo, I wouldn't mind helping with your mod - I'm not sure how much time I will have, but still - any extra help is a good thing, right? Just drop me a PM sometime.
Apollonius
"Success isn't how far you got, but the distance you traveled from where you started."
- greek proverb
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