It's quite possible that the purpose of the ditch was to provide soil for the bank! Once you defined your 'sacred goat enclosure' on the ground you wouldn't want to pile the soil up inside it would you :-)
Fill 'em with water and create an island inside. Myths are full of legends of far away magical islands. Could they have been the structures currently known as henges? Or could the henge be a representatin of such a place?