It was situated near the entrance of the eastern pass across Mount Parnes, which leads from the northeastern part of the Athenian plain to Oropus, and from thence both to Tanagra on the one hand, and to Delium and Chalcis on the other.
With advice from Alcibiades in 415 BC, the former Athenian general wanted on Athenian charges of religious crimes, the Spartans and their allies, under king Agis II, fortified Decelea as a major military post in the later stage of the Peloponnesian War, giving them control of rural Attica and cutting off the primary land route for food imports.
Spartan patrols through the Attic countryside strained the Athenian cavalry and curtailed the ability of Athens to continue exploiting the Laurium silver mines in southeastern Attica that were an important source of income.
Thucydides estimated [4] that 20,000 slaves, many of them skilled workers, escaped to Decelea, from 413 until the close of the Peloponnesian War in 404 BC.
The site controls what was once a major ancient road, usable by carts, connecting Athens to the grain port of Oropus.