EDIT: Forgive me, I’m cranky. Of course the communities resting on Reddit’s beach are themselves the castles. I’m so tunnel-visioned on the API aspect that only used that lens.
The sandcastles are the communities built around subreddits. The subreddits are on a corporately owned beach, the owners are building a mall, and the sandcastles are in the way.
EDIT: Forgive me, I’m cranky. Of course the communities resting on Reddit’s beach are themselves the castles. I’m so tunnel-visioned on the API aspect that only used that lens.