If it was a fort, an internal ditch would be a bit silly. Even a stock enclosure would have the ditch on the outside. General principle for stock-proofing is you dig your ditch and throw the soil up on the inside of it. Otherwise your cows fall in the ditch and get stuck.

Not sure why there would be an internal and external ditch?

Looks like a good shout for a henge or hengiform.