The Centurion is the ultimate elite infantry unit. It has more hit points, attack strength, and armor than the Phalanx. You must research Aristocracy before you can upgrade to the Centurion.
Researching Toolworking, Metalworking, and Metallurgy increases attack strength. Leather Armor, Scale Armor, and Chain Mail increase armor. The Bronze Shield and Iron Shield increase piercing armor. Aristocracy increases speed.