Natural stone that carved into smooth walls = un-climb-able.
Constructed blocks (stone, logs, metal) made into smooth walls = difficult to climb.
Stone and logs constructed into rough walls = climb-able.
Natural dirt and stone walls = climb-able.
Enemies can jump z+1 level, so enemies can jump z+1 be on the 2nd floor of the wall.
A 1 z high rough wall, with a 1 z deep dirt dry moat.
Enemies can jump down to the dirt moat, jump up z+1 and hold onto the rough wall, proceed to climb the rough wall and be on the 2nd floor.
A mason's block, can be constructed into Fortification. The result is 1 fortification on the z+0, but no floor on the z+1.
A mason's block, can be constructed into a Wall, then carved with Fortification. The result is 1 fortification on the z+0, and has a floor on the z+1.
A 1 z high smooth wall made from blocks is difficult to climb.
Enemies can jump z+1 level, so even with difficult to climb smooth wall, enemies can jump z+1 and be on the 2nd floor.
A 1 z high smooth wall made from blocks is difficult to climb with a fortification on z+2.
Enemies can jump z+1 level even with difficult to climb smooth wall, enemies can garb the fortification on the z+2 and climb that to go over the wall.
Fortifications. There are cases where the dwarf dodges successfully, but ends up going through a fortification. Happens a lot with barracks designation including walls.