Completely disordered among what's left of the castle (2,5)
I believe the answer is:
in ruins
'completely disordered' is the definition.
I know nothing about this answer so I can't tell whether this works.
'among what's left of the castle' is the wordplay.
'among' becomes 'in' (eg among or in a group).
'what's left of the castle' becomes 'ruins' (I can't justify this - if you can you should give a lot more credence to this answer).
'in'+'ruins'='IN RUINS'
Can you help me to learn more?
(Other definitions for in ruins that I've seen before include "Wrecked" , "Ramshackle" , "falling apart" , "Broken-down" , "Effectively destroyed" .)