In regards to Ancient Empires, I can't quite find the rule for Disordered (or disorganized or disciplined). It's a modifier on page 40, but I can't find the rules how being 'disordered' occurs?
I don't check BGG as much during the school year as I'm usually way too busy. Good thing I looked.
The rule you are searching is on page 39 under Movement. It's in bold print saying "You may move full move downhill, but the unit is disordered. That actually would apply to any obstacle, such as moving full move across a stream.
And while it's not written in the rules specifically, you may expand the situations that lead to disorganization by including fighting in thick woods, confined towns, standing in a broad stream. Logic would indicate that the terrain would inhibit an organized formation. Also, even though The Games of War is a compendium of rules sets, it still is one book and you can feel free to add to any rules set some aspect of another.
For example, "pop ups" are usually associated with WWII or later games, but could just as easily be applied to any era such as ancients. You might allow barbarians some units that they may pop up in their movement that their opponents had not seen and hadn't expected. Ambushes and fog of war are era friendly.
I hope that helps.