Q. What is Caerphilly cheese?
A. Caerphilly is a cheddar-type of cheese, originally produced in Wales, but mostly made now across the border in England. It is a crumbly, firm, white, cow's milk cheese, with a slightly salty flavor. It is not aged for long (one to two months), and is good for melting.
If you don't have a chunk of it sitting in your refrigerator, Lancashire and the French Cantal are close substitutes.