Behold, when we come into the land, you shall bind this line of scarlet thread in the window which you did let us down by: and you shall gather to you into the house your father, and your mother, and your brothers, and all your father's household.
That "scarlet thread" so prominent in this narrative was surely a very strong and efficient rope, capable of carrying the weight of a man.  A red rope is certainly unusual, and we cannot entirely overlook the connotation throughout history of the color red, often associated with brothels.  "The red light district" is an expression still known in many places.