A Citadel is an idea from an old Reddit post about a gated community built out of necessity for bitcoiners. ASIC miners, armed personal protection, self-sustaining systems might be needed to protect the wealth of the new rich from the hoi polloi, the unwashed masses of the nocoiners. It’s also very desirable for some to think of a utopian community of like-minded individuals living in harmony. Realistically, a citadel would provide a single point of failure so it’s unlikely that we’ll see anything forming as originally envisioned. Decentralization and pseudonymity is bitcoin’s strength and the same goes for its users.