Thermopylae was the only gateway into Greece for the Persian army, and presented the perfect choke point — a narrow pass bordered by a huge mountain wall on one side and a cliff drop-off to the sea on the other. This area decreased the Persians' advantage of having large numbers. Delaying the Persian advance here would give the Greek allies enough time to ready a larger, main force to defend against the Persians. The battle takes place simultaneously with the sea battle at Artemisium, where the Allied Greek forces hoped to protect the flank of the army at Thermopylae whilst not being cut off themselves. The Greeks were at a disadvantage at Artemisium, as at Thermopylae - the Persians outnumbered the Allies, and most of the Athenian ships were newly built and manned by inexperienced crews - and both sides suffered heavy losses in the sea battle.