A three-day festival held each year in late July on the green between the castle and the town, dedicated to Caerphilly cheese and to a slightly anarchic mix of medieval re-enactment, falconry, fire shows, Welsh choral singing and craft stalls. Free to enter, around 80,000 people attend across the weekend, and the town effectively closes down to make room.
If your visit can be timed to coincide with it, do so — it's one of the best small-town festivals in South Wales. Outside of that weekend, the festival is invisible; the rest of the cheese-related action is at the indoor market and the dairy on Pwll-y-pant Road.