The basic idea is to be able to make walls path-able and climbable for anyone (probably barring some disability) under normal circumstances, just like stairs or ramps. The reason I'd like to carve these into walls is because I don't want there to be a limit to how high they can go and with a movable ladder there will be more limitations.
There are a couple reasons I'd like this, I tend to build in caverns and I think it would be a good way of restricting traffic in and out (because you can't lay down to let another person climb over you), and I think it would simplify pathways in complicated multi z-level natural areas. I also think some ladders here and there would look more natural and ascetically pleasing feel than digging stairs into the walls.
It would open up creative new defense measures, like forcing a sieging army to climb ladders down a deep pit to enter your fortress while being exposed to marksdwarves, and it would also help squads stay in a position that is more easily defended. Right now I usually place a squad at the bottom of a staircase the siege is descending, but as soon as the first goblin pops out the dwarves run up the stairs in to the mix, with ladders they would be more forced to hold the positions
Bonus - useful for elven treehouses though I guess they'd 'grow' the ladders into the trunks instead of carving them.
I imagine it working like any engraving/fortification, the people are just stuck to the wall standing over air. No need for downladder/upladder stuff, and they should be able use the ladder below or above to carve the next level.
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