くるわ

a palisade, wall, moat, natural waterway, or other structure used to divide a specific area from its surroundings, such as a castle or fort

the area so bounded by such a structure

a red-light district, specifically one that is divided from the rest of the town by means of a wall or fence or similar structure

(poetry, haiku) in 俳諧 (haikai, “vulgar haiku”), the general realm of subject matter used for the opening line