Some sort of climbing apparatus is planned but the implementation of this mechanic would be nice to see, do you mean abseiling using rope from your inventory to make temporary climbing tiles in mid air? (i guess that's a simplification) because that's how i'd practically see it happening personally.
- Not forgetting siege ladders or enemies throwing their ropes over the top of your fortifications to make climbing tiles upwards
On that note about expanding climbing natural plant formations that are attached to the side of a building or a wall that grow naturally over time would also be nice obstacles to clamber over, a strategic problem that may allow a easy point of access into your fortress.
That's how I would imagine it working too. It takes one piece of rope out of your inventory and turns it into a climbable tile, much like how placing rope works in Terraria.
And about enemies throwing ropes over walls, maybe there could be an item called a grappling hook, and when you/an invader throws it it creates a tile where it lands with rope going as far down as you have/physically possible.
Now I'm imagining specially trained "climbing goblins" who try scaling your block walls and dropping down ropes when they reach the top for the regular gobbos to climb up.
Who wouldn't love watching an army of goblins funnel up a long rope on the side of your wall, only to all fall after your axedwarf cuts it lose?