If you look at modern paintings and miniatures, you would think we have a good idea of the type of shield used by Achaemenid infantry in the time of Darius and Xerxes. They cite Herodotus book 7 chapter 61 and show the large rectangular kind on the middle row of the picture above. But as I argue in chapter 6.5.2 of my forthcoming book from Franz Steiner Verlag, things are more complicated. These large rectangular shields appear on the doorposts of two buildings at Persepolis and on two or three vases from Athens (out of thousands of soldiers at Persepolis and Susa and thousands of Red Figure vases). The person who published the sketch on the middle left thought it showed a battle against the Phrygian allies of the Amazons. And this type of shield does not agree with Herodotus’ words that quivers were hanging beneath the shields, unless we understand ‘beneath’ quite loosely.
The type in the top row is also popular with artists, but it seems specific to the Aegean/Sea of Marmara/Black Sea region. Warriors of all the different nations in this region used small round, rectangular, or crescent-shaped shields, often made of wicker or covered with goatskin, but we do not see these shields in artwork from the heart of the empire. Its also hard to imagine these forming a barrier which needed to be knocked over after a long struggle, like Herodotus describes. But they are the type of shield which artists from Athens most often put in the hands of foreigners, probably because the Athenians were a bit provincial and Thracians and Scythians were the most exotic warriors they were familiar with.
The type in the bottom row is the most common type at Persepolis, but it does not show up in Greek art at all. This might be because it was too similar to the Boeotian or dipylon or figure-eight shield and Greek artists felt that it did not say ‘barbarian.’ A bronze boss for such a shield was found at Samos. It is easy to imagine quivers hanging beneath such a shield, but hard to imagine them forming a solid barrier which needed to be pushed over.
We see even more types of shield in art from the empire, such as the small round center-grip shield on the Çan sarcophagus or the big deep round wicker shields carried by ?Thracians? at Persepolis. But these three types in the three rows are the ones people today are most interested in.
Diodorus describes the Persian shields at Thermopylae a bit differently than Herodotus describes them at Plataia (11.7.3, my translation) “For the barbarians used small aspides and peltai which were advantageous in broad open spaces, being easy to move, so because of the narrow space they were not able to harm their enemies very well because they were packed close together with their whole bodies covered with the great aspides, while being at a disadvantage because of the lightness of their defensive arms, they suffered wound after wound.” He seems to imagine Persian warriors using peltai like the barbarians in paintings (Xen. Cyr. 1.2.13)
People in my lifetime have written a lot of nonsense about wicker shields supposedly being ‘light’ or ‘weak.’ I don’t know of any tests of this, and traditional wooden shields tend to be thinner and lighter than people expect (like 8 mm thick wood plus a layer of linen or parchment in the centre, and 3-5 mm thick wood plus a layer of linen or parchment at the edge). Modern wooden shields often use heavy plywood and rawhide so the shield can stand up to pounding with blunt weapons: the fact that these shields are unnecessarily heavy is not important because the owners don’t have to march 20 miles with them on their backs, camp, cook dinner, and march 20 miles again. Tough warriors like the Ottoman Turks used wicker shields and I don’t know of any of their opponents who said that these were poor protection. Also, until about the time of Xerxes’ punitive expedition against the Ionians-beyond-the-sea, Greek art often shows spearmen with a light figure-eight shields (Dipylon shields, Boeotian shields) which may well have been made of leather or wicker. Neither art not archaeology shows that all Greek “bearers of spear and shield” in 480 BCE carried a heavy Argive shield and a single long thrusting lance.
The type of shield in the middle row is the best fit for Herodotus’ descriptions of combat, but not the most common type in art, and it does not match his description of how Persian soldiers were armed. And as we saw, lists of equipment for soldiers in Babylonia do not seem to mention shields at all. So like a lot of things in ancient history, the idea that Xerxes’ men carried large rectangular wicker shields is our best guess based on pretty thin evidence, not something certain.
Further Reading: There are tests of arrows against reproductions of sticks-through-rawhide shields based on artefacts from Dura Europos and Pazyrzk in Barry Molloy’s The Cutting Edge (Tempus: Stroud, 2007) [ISBN 9780752441696 on Bookfinder]