Was it deliberately filled with water originally? I don't know. It would need to be deliberately lined with clay but how can that be known when clay would tend to accumulate over time anyway?

I don't think crocodiles in the moat are the answer as being eaten by one of them might be interpreted as an unlucky accident whereas a slap from Goff could be mentioned on EH's notices as a justified consequence of flouting their request to keep off the hill. He'd be very effective at Stonehenge too, he could hide behind a stone, he's good at that.