Archaeologically, it's rather interesting, because this type of shield seems to have been used continuously from pre-historic times to Archaic period in the Aegean:

📸 1. Minoan ring depicting a Dipylon-type shield (~1550 BC)

📸 2. Mycenaean medallion depicting a Dipylon-type shield (~1300 BC)

📸 3. Crater from Dipylon, Athens, decorated with scenes depicting warriors with the characteristic shields (~750 BC)