くるわ

Etymology
Compound of, referring to a wall, palisade, moat (often dry), or natural waterway surrounding a castle or other fortification.

First appears in the late 1500s in reference to such a wall or moat. Later used to refer to the area bounded by such a feature, and then used periphrastically to refer to a red-light district, which was usually divided from the rest of a town by such a wall or fence.

Noun

 * 1)  a palisade, wall, moat, natural waterway, or other structure used to divide a specific area from its surroundings, such as a castle or fort
 * 2)  the area so bounded by such a structure
 * 3)  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
 * 4)   in, the general realm of subject matter used for the opening line
 * 1)   in, the general realm of subject matter used for the opening line
 * 1)   in, the general realm of subject matter used for the opening line