Edge cache tiering and purge

Design a content-delivery edge cache hierarchy for a global service serving static assets and large media: hundreds of edge POPs near users, a smaller tier of regional 'mid-tier' caches, and a small set of origins. Goal is a high offload ratio (most requests never reach origin), low latency, and the ability to purge content globally within seconds. How do you structure the cache tiers, route requests, and handle invalidation and origin protection?