Fred the Labrabor took the baby orphans under his paw after their mum vanished on Thursday.
The 10-year-old pooch is the resident pet at Mountfitchet Castle near Stansted, Essex, where a member of staff found the lost siblings waddling around the battlements.
Staff at the popular tourist attraction said they were very worried about the ducklings, but Fred immediately took to them and has been babysitting ever since.
Jeremy Goldsmith, director of Mountfitchet Castle, said: "We knew the mother duck was nesting and then suddenly these ducklings appeared but she was nowhere to be seen.
"We think it was possibly a fox."
Adorable photos show Fred lying on the grass in the grounds of the castle, his adopted children nestling by his side.
In one picture, the nine brothers and sisters can be seen sitting on his back.
A member of staff said: "They are getting very mischievous even hitching a ride on his back and following him into the water for a swim in the castle moat."
Mountfitchet Castle is a Norman Motte and Bailey castle, built of wood.
Formerly an Iron Age hill fort, the historic site was also a Roman signals fort and later a Saxon and Viking settlement before being attacked in 1066 by William the Conqueror.