The free population could have been little short of twenty thousand.

We must add the numbers, already great, of the resident aliens and the slaves, who, as Pausanias tells us, were then for the first time admitted to military service.

On the other hand it is evident, from the speech of Miltiades to Callimachus, and the supposed treachery of the Alcmaeonidae, that some, nor an inconsiderable, force, was left in reserve at Athens for the protection of the city.

Let us suppose, however, that two thirds of the Athenian citizens of military age, viz., between the ages of twenty and sixty, marched to Marathon (and this was but the common proportion on common occasions), the total force, with the slaves, the settlers, and the Plataean auxiliaries, could not amount to less than fifteen or sixteen thousand.

But whatever the precise number of the heroes of Marathon, we have ample testimony for the general fact that it was so trifling when compared with the Persian armament, as almost to justify the exaggeration of later writers.
[278] Plut.