The Greek nation has the Phalanx as its unique unit, replacing the Speaman. The Phalanx is superior to Spearman in terms of strength (5 versus 4), making it a serious threat in the early stages of the game.
Until Axemen are built this is a most powerful unit. Guarding cities with Phalanx is a good idea as well as it has a +100% versus mounted units (common to Speaman) – making it closer to Pikeman, which requires Feudalism, a relatively advanced technology. Phalanx also has +25% hills defense. As Hunting is the Greek starting technology, Phalanx can be built as soon as copper is discovered (research Bronze working).
The starting technologies for the Greeks are Hunting and Fishing. This means that Archery, Animal Husbandry and Sailing can be researched straight away. Mining should be given a priority to enable Bronze Working - which will give you the unique Phalanx.
The civilization has only 1 leader – Alexander.