It's not so much that there's a "downhill" both up and back, but rather that the plateaus are not at all level with each other, meaning that adjacent plateaus may each have higher and lower points. You don't cross over at the same place each way. One reason that Sadeas's bridge crews are faster and more effective than Dalinar's mobile bridge towers is that it's easier to direct 25 men carrying a 40-50' bridge over rough terrain than to tow bridge towers in relays.
I understand that, but then they wouldn't be able to arrive exactly back in Sadeas' camp every time, or at least it doesn't make much sense.
- If they leave over Plateau A and want to get to Plateau B, then logically Plateau A will be higher than B.
- To get to Plateau C, B will have to be higher than C
- To progress, they would always have to find a lower plateau.
- Eventually, they have to come back to platform A, or some other Plateau, which will be higher than any Plateau around it, since the path they take gets continually lower and lower. They literally cannot keep finding plateaus that go continually lower and expect to wind all the way up at the camp again.
Wouldn't they eventually come to a point where they would HAVE to go upward? Unless you're going to say "well the land is just so uneven that there's always a way to go down that will lead to a way up" or "there are some plateaus that have a low end and a high end." Which I can buy, but just how many?
But then, this conundrum can be oh-so-easily explained by the permanent Bridges, couldn't it?
Still, I think there would come a point where it would be impossible to reach certain plateaus or return to certain camps. Interesting thought.