Castle in France with turrets is periodically housing soldiers (8)
I believe the answer is:
fortress
'castle' is the definition.
(I've seen this before)
'france with turrets is periodically housing soldiers' is the wordplay.
'france' becomes 'f'.
'with' means one lot of letters go next to another.
'periodically' means one should take alternating letters (letters must be selected 'every so often').
'housing' is an insertion indicator.
'soldiers' becomes 'OR' (Other Rank).
'turrets'+'is'='turretsis'
The alternate letters of 'turretsis' are 'tress'.
'f'+'tress'='ftress'
'ftress' going around 'or' is 'FORTRESS'.
'in' acts as a link.
(Other definitions for fortress that I've seen before include "Perhaps keep" , "Castle" , "Protective structure" , "Military stronghold" , "Strong point" .)