The Battle

The two armies faced each other for perhaps four days. Each side had good reason to wait. For the Athenians, each day that passed brought Spartan aid closer - with the end of the Carneia on 12 August, the Spartans would be able to march to their aid arriving perhaps by the 15th. Moreover, given the large expanse of the Plain of Marathon, and the ability of the Persians to deploy and manoeuvre their cavalry there, it did not make tactical sense for the Greeks to march out of the favourable terrain near their fortified camp which was situated between the sea and the hills. This would make the heavily-armed hoplites, in their phalanx formation, a formidable force against the more lightly-equipped Persians. For their part, the Persians also had reason to believe that time was on their side, at least initially. As had occurred in Eretria, they hoped for help from within the city itself. In this case, they expected the supporters of Hippias to betray the city to them, no doubt with the encouragement of Persian gold. The Persians were waiting for signal, a highly polished bronze shield would be flashed from the Mount Pentele which would indicate that all was ready.
While the details of the plan are not known, it seems clear that the Persians would embark the bulk of their troops on ships and sail to Athens while the Athenian forces were still at Marathon. Moreover, the Persians did not wish to assault the Athenians and Plataeans while in their strong position at the southern end of the plain since the terrain would nullify both their numerical advantage and the mobility of their cavalry.
By the evening of 11 August, however, time was running out for the Persians. There had been no shield signal from the fifth column in Athens and the Spartan festival would be coming to an end shortly. That meant that the Athenians could expect Spartan reinforcements and the presence of such tough, well-disciplined hoplites would dramatically transform the military balance on the Plain of Marathon. As a result, the Persians began to embark a part of their forces on the transports while the remainder of their forces kept a watch on the Athenian and Plataean hoplites at Marathon. Even without the shield signal, the Persians could hope for help from within the city if the army was away.
This force was to be under the command of Datis and seems to have included the majority of the cavalry who would be very useful in making a dash for Athens once the task force made landfall at Phaleron Bay. Artaphernes would stay at Marathon and mantain a close blockade of the Athenian camp. He probably had about 15,000 men with him, almost exclusively infantry. Fortunately for the Athenians, they were alerted to the Persian plan by some sympathetic Ionians who were serving with the Persians. They sent the famous message 'the cavalry are away' which galvanized the resolve of the Athenian commanders to offer battle. Indeed, the 10 Athenian strategoi, generals elected from each of Athens' 10 tribal divisions, were deadlocked as to whether or not to stay put and fight, return to Athens, as they, too, were mindful of treachery from within the city, or to offer battle, a course of action favoured by Militiades.

Persian Armoured Cavalry- Achaemenid era