What is Citadel?
Citadel is an anti-grief plugin, built for multiplayer Minecraft servers. To use Citadel, simply hit a block with a reinforcing material (like a stone block, or a diamond). This will lock the chest or block and make it so others cannot easily break it, or use it until it is broken. People can still get into your chests if they are locked, but it requires raiders to break the same block until the reinforcement runs out.