The Classical era (600 BC to 500 AD) is one of the extra eras that came shipped with the game in addition to the 5 default ones. The additional Era besides this is World War 2. This era is considered by the game designers to be a subset of the Ancient age and World War 2 was part of the Modern and pre-Genetic eras. Historically most of the Ancient age takes place in the Classical era (events wise not length wise). Due to the fact that this era lasted for 1100 years its considered to really be an epoch rather than all one era. Game players that have found themselves in a Classical era scenario will need to learn new strategies and ways to wage warfare to complete their objectives.