earthwork enclosure around a castle
郛 refers to an outer earthwork or wall that encloses a castle, forming a defensive perimeter.
郛 combines ⻏ (village, walled settlement) with 孚, which likely contributes a phonetic element. The character originally denoted an outer defensive wall around a city or castle.
The left side 孚 suggests something enclosing or protecting, and the right side ⻏ represents a walled place. Together they form the outer earthwork that surrounds a castle.
For フ, imagine the outer wall is so strong it makes a deep 'foo' sound when struck: foo -> フ, echoing around the castle enclosure.