In 413, on the suggestion of Alcibiades, he fortified Decelea in Attica, where he remained directing operations until, after the battle of Aegospotami (405), he took the leading part in the blockade of Athens, which was ended in spring 404 by the surrender of the city.
The surrounding rampart of mountains was broken towards the north-east by an open tract stretching between Hymettus and Pentelicus towards Marathon, and was traversed by the passes of Decelea, Phyle and Daphne on the north and north-west, but the distance between these natural passages and the city was sufficient to obviate the danger of surprise by an invading land force.
On the advice of Alcibiades (q.v.), exiled from Athens in 415, they had fortified Decelea in Attica within fifteen miles of Athens.
In 412 many Ionian towns revolted, and appealed either to Agis at Decelea or to Sparta direct.
The rocky promontory on which the temple stands was fortified by a wall with towers, in 413 B.C., as a protection against the Spartans in Decelea; but it was soon after seized by a body of fugitive slaves from the Laurium mines.