I did a few custom battles with different factions and noticed that different cultures have different formations. Hellenic factions will have some pike centric formations with light infantry and cav on the flanks, Germanic tribes have very hard hitting formations, with lots of heavy infantry in the center while with Rome, you can actually put them in the formation they used before the state started equipping its citizen infantry and they ended up being equipped in largely the same way.

It is kind of neat.