Group of nesting birds in castle on European lines (7)
I believe the answer is:
rookery
'group of nesting birds' is the definition.
(group of rooks)
'castle on european lines' is the wordplay.
'castle' becomes 'rook' (synonyms).
'on' means one lot of letters go next to another.
'european' becomes 'E' (abbreviation e.g. EU).
'lines' becomes 'ry' (abbreviation for railway).
'rook'+'e'+'ry'='ROOKERY'
'in' is the link.
(Other definitions for rookery that I've seen before include "Noisy bird colony" , "Lofty breeding-place" , "Breeding colony of birds, also of seals or turtles" , "Seal, penguin, colony" , "Community of birds" .)