Preventing climbers into fortress

Consider putting traps along the top of the floor level (along the outer edge) causing anyone to try and dodge, and hopefully dodge the 'wrong' way causing them to fall off (increase this by building a wall one step 'inwards'.

Another step you can take is to add fortifications, this has two advantages:

• You can have marks dwarfs firing through the fortifications in relative safely
• Fortifications can not be stood on unless they have a floor tile on top. This should stop anyone trying to cross over them. Due to jumpers, you may want multiple rows of fortifications.

Repeat as many times as necessary ;)

Some animals are legendary climbers (cats are by default, and Giant Cave Spiders can move at 'walking speed' when climbing); so if they can swim as well then you're current defence would possibly still fail to them.

Also this defence does nothing to stop flying coughDragoncough enemies from entering your fort from above. The only way to truly stop anyone 'climbing' into your fort is to seal the roof off. After all, who wants the 'orrible glowing sky fire shining on them while they work anyway?!

Climbing smoothed natural stone is impossible.
So smooth your walls from the inside. Note that carved fortifications are carved through a smoothed wall, and constructed fortifications are considered smoothed also. If you REALLY want to prevent climbers and flyers, build a floor over your fort. Gaseous creatures might still enter through your fortifications though.
Beware of jumping creatures. They will jump over your moat, even if it's filled with magma, then grab onto a rough wall.
Ghosts will just fly through anything, including solid rock.

Edit: I'm pretty sure an overhang, like in your picture, prevents climbing too. However, enemies could just climb around your walls and moat, on the rock face against which you built.