The ancient site of Aphidna is found just east of Kosmothea in north-east Attica, close to the plain of Marathon. A Classical fortification is situated on the Kotroni hill, while the Charadra stream is located just below and to the south-west with a Middle Bronze Age tumulus on the southern side. A second stream, the Varnavas, flows about 1.5 km to the east. In the late 19th century both rivers emptied into Lake Marathon c. 2.5 km away; today, the landscape has been radically changed as a dam has increased the size of the lake almost to the Kotroni hill.
Aphidna was in Classical times one of the Attic demes and an important fort protecting the northern Attic border. The fortress is situated on the acropolis of Kotroni on the northwestern edge of the Marathon Lake.