Within ancient tribal groups an early hierarchical structure centered around the strongman, who probably took power in a physical contest, led the group, and enjoyed special privileges. As populations increased, the hierarchy expanded. Layers of nobility, a class of society privileged due to fighting prowess or wealth, grew between the stronghold, or king, and common people and slaves. The nobility served as administrators and sub- commanders of the army. Examples of nobility were the Persian satraps, who ruled provinces of the Persian Empire, and Alexander the Great's Companion's, who commanded parts of his army and formed the core of his heavy cavalry squadrons.