History of the Peloponnesian War

On the night in question the cart was already at the gates, and when these were opened as usual as if to let the boat pass through, the Athenians, who were acting throughout in accordance with an agreement, seeing it, ran at top speed from their ambush, wishing to get there before the gates were closed again and while the cart was still in the passage, thus forming an obstacle to the shutting of the gates; and at the same time their Megarian accomplices killed the guards at the gates.

And first the Plataeans and the patrols under Demosthenes' command rushed into the place where the trophy now stands, and as soon as they were inside the gates the Plataeans engaged with the reinforcements which came up—for the nearest Peloponnesians had become aware of what was going on—and defeated them, thus securing the gates for the onrushing Athenian hoplites.

After that every Athenian who got inside immediately made for the wall.

A few of the Peloponnesian garrison at first stood their ground and defended themselves, some of them being killed, but most of them took to flight, being seized with panic, both because the enemy had attacked them at night, and also because they thought the Megarian traitors were fighting against them; and they supposed that all the Megarians had betrayed them.