Power!!

A Citadel holds troops and allows for increased trap counts and march sizes, it has NO REAL POWER itself.

Real Power comes from BOOSTS! Hero Boosts follow your hero. If your Hero is at home, your empire gets it boosts. If it is in an attack march or rally, that march or rally receives it's boosts. That makes your empire vulnerable when your hero is away or captured.
 * 1) Get boosts by maximizing Combat and Defense Research for your Citadel Level
 * 2) Craft and Upgrade Hero Gear, Sentinel Gear has tremendous defense boosts!

When targeting Big Players - Scout him and look at his Hero Gear - that will reveal his true POWER!!