Although situated on a plain, Athens is surrounded by mountains: Hymettos and Pentelikon in the east, Aigaleos in the west, and Parnes in the north. Decelea is situated in a pass in the Parnes range, near the source of the Cephisus river, and - according to the Athenian historian Thucydides - 120 stadia (21 kilometer) north of Athens. It controlled one of the main roads from Thebes to Athens; in 479, for example, the Persian army of Mardonius passed through Decelea when it reoccupied Athens.
The town is mentioned a couple of times in our sources. We know that Decelean wine was considered to be useful only as vinegar. The town had sanctuaries for Zeus and Leto, and was believed to have been founded by an autochthonous hero named Decelus.
The Greek researcher Herodotus of Halicarnassus tells an Athenian story about Decelea:
Castor and Polydeuces invaded Attica with a great host to bring home Helen [who had been captured by the Athenian king Theseus], and laid waste all villages, because they did not where she was held captive. The Athenians say that the Deceleans, or perhaps Decelus himself, being aggrieved by the insolence of Theseus and fearing for all the land of the Athenians, told them the whole matter and led them to Aphidnae, which was betrayed [...] to Castor and Polydeuces.
In consequence of this deed the Deceleans have always enjoyed freedom from dues in Sparta and front seats at the games, privileges which exist still to the present day. Even during the war between the Athenians and the Peloponnesians that broke out many years after these events, when the Spartans laid waste all the rest of Attica, they abstained from injury to Decelea.note[Herodotus, Histories 9.73.]
This tale may have been told in a sanctuary dedicated to the legendary Twins, who were also venerated in Sparta. In the present version, the story must have circulated between 431, when the Archidamian War and the annual Spartan invasion of Attica started, and 425, when the invaders were forced to give up these campaigns because the Athenians had captured 292 Spartans at Sphacteria.
In the spring of 413, the Spartan king Agis II occupied and fortified Decelea, an act of strategic and symbolic importance. From a strategic point of view, it meant that they controlled one of the main roads in Attica. The Athenians were no longer able to take the route across the Parnes to bring food supplies from Euboea to their city. At the same time, the Spartans - who belonged to an ethnic group that was called Dorian - by occupying this lieu de mémoire, symbolically exploited an internal division within the ethnic group to which the Athenians belonged, the Ionians.
The capture and fortification of Decelea marked the beginning of the second phase of the Peloponnesian War, which is called the Decelean War. During its first phase, the Spartans had laid waste Attica with brief raids; now, they were to stay permanently, and received help from the nearby Thebans. Thousands of slaves ran away, and the Athenian economy was crippled. The author of the Oxyrhynchus Hellenica writes:
Easily, the Thebans seized captives and other war spoils, which they carried off to their homes. As they inhabited a nearby country, they even carried away the furnishing material, beginning with the wood and tiles of the houses of Attica.note[Oxyrhynchus Hellenica 12.4.]
Thucydides says that the Spartans got the idea to occupy Decelea from the Athenian turncoat Alcibiades, but it is unlikely that they had not discovered its importance themselves.