Gla was an important fortified site of the Mycenaean civilization of Ancient Greece, located in Boeotia. The site formed an island within Lake Kopais, and it was founded by the Boeotians after their expulsion from Thessaly; at the time of the Trojan War, the city was known as Arne. Gla was ten times larger than contemporary Athens or Tiryns before Lake Kopais swallowed up much of the city, and, during the Peloponnesian War, the Athenian military transformed Gla's ruins into a fortress.