Maybe the answer is all of the above. The Achaemenid empire was a massive empire, stretching from the Danube to the Indus river, from the steppes of southern R

Maybe the answer is all of the above. The Achaemenid empire was a massive empire, stretching from the Danube to the Indus river, from the steppes of southern Russia in the north, to the Nubian desert in the south. At its height, it probably ruled over 45% of the world's population, something no empire has ever reached since. There are 4 rivers systems that are the cradles of ancient civilization: Nile, Tigris/Euphrates, Indus, Yellow. The Achaemenid empire controlled 3 of the 4 at its height: Nile, Tigris/Euphrates, Indus. Such a vast empire, drew its military from all over. Many ethnic units fought with their traditional weaponry. In addition, such a large army had it's specialized units from the cavalry to the elite Immortals. Each were equipped to maximize their effectiveness. Thus, it is likely that all these types of shields were relatively common depending on the battle. Perhaps different shields were used by different soldiers in different configurations or battles or terrain. One thing though. For all its might and power, the Achaemenid empire found itself curiously vulnerable the Greek Hoplite and later the Macedonian Phalanx. Such are the vagaries of history.