Strong places in which to fling a fellow (7)

I believe the answer is:
castles
'strong places' is the definition.
The answer and definition can be both acts as well as being plural nouns.
Perhaps there's a link between them I don't understand?
'to fling a fellow' is the wordplay.
'to fling' becomes 'cast' (I've seen this in another clue).
'a fellow' becomes 'les' (Les is a man's name).
'cast'+'les'='CASTLES'
'in which' is the link.
Can you help me to learn more?
(Other definitions for castles that I've seen before include "big houses" , "buildings" , "Keeps" , "Men" , "Fortresses" .)
