It's not conquering. It comes from Polis + Herkos, meaning "closing the city", ie laying siege. "Poliorcétique" is the techniques of siege.
French has a lot of greek roots, and many latin of course.
Somwhat similar meaning, although i was not conscious of the words it breaks up to, thanks

Infact "Herkos" seems to have dissappeared from use, at least at this moment i cannot think of any other word with that root.