Most of our scenary is for Mordheim, and therefore has ladders and stairs built into to some of them (and loose ladders available), so I think that as FG allows any surface to be climbed at half-movement, ladders and stairs should be climbable at normal movement.
On one hand, you could argue they're as decrepit as the rest of Frostgrave, hence the movement reduction; on the other hand allowing normal movement can made it a tactical decision on whether to climb a wall slower or risk being in a bad position but moving quicker (in my game moving up some stairs would have allowed me to collect a treasure but only being behind one set of cover, whereas climbing a wall would have taken two turns, but kept me at -3 all the way).
I asked the question on the Frostgrave forum, and most people are of the view of full movement, but I thought I'd discuss here, if only so we can get a consensus sorted before it comes up as an issue in game.