A member of medieval law enforcement, the Constable was given charge over certain parts of the King's army. He is also considered the Captain of the Guard. Modern day use of the term continues primarily in the United Kingdom's police force.
The Constable polices the Villages universe, working with the Knights of the Realm to provide justice for all.
Obviously, the ability to gain an extra power from a Knight in its village makes the Constable raise his power to above average levels. He is also more powerful against hired units, so many units with a hiring bonus will still lose to him.
Be wary, though; since if he loses he can't be kidnapped, your opponent will certainly take the opportunity to destroy him, costing you 3 gold and future defense capability.