Answer:
Acton Burnell was originally a small fortified manor house, probably with a moat, an earth bank and timber palisade around its perimeter. Much later it was adapted, altered and enlarged as a piece of landscape architecture or folly - it was never a "castle" in the proper sense. Many places were called a "castle" in the 17th and 18th centuries as a kind of status symbol, even though they were never actually castles.
No trace remains today of the earth-and-timber external wall, so it is impossible to say how large the manor complex was in the 13th century, when it was first built.