Where does “Caerffili” come from?

Caerffili (Welsh) comes from Welsh caer, from Proto-Celtic *kaɸerūxs — sheep.

Ancestry of “Caerffili”, step by step

Step	Language	Word	Meaning
1	Welsh	caer	fort, fortress, enclosed stronghold, castle,...
2	Proto-Celtic	*kaɸerūxs	sheep