Panel 15: In the fifteenth panel of the "Capture of the city of Jericho," thanks to the miraculous fall of the walls at the sounding of the trumpets, Rahab reappears according to the agreement made with the explorers. They had agreed that she was to be saved along with her family from the sack and destruction of the city if, at the moment of the capture, she had hung from her house the red rope by which the explorers had earlier been able to climb down from a window of her house, which was built into the city wall, and to escape from the city into the hills (Joshua 2:15-21; 6:17-25). The mosaic representation does not strictly follow the Biblical narrative, but the red sign which appears in the vault of the city gate, above which appears Rahab at the moment in which the army is about to enter the defenseless city, signals the existence of the pact of which she was the subject and to which the mosaic narrative makes no other reference. This necessitated making a tight connection between Rahab, the scenes of the explorers' flight from Jericho, and the subsequent fall of the city. This change in the Biblical account required postponing the scene below (Joshua 6:12-19), which shows the "Procession of the Ark of the Covenant" to the sound of the trumpets around the city. Obviously, this happened not after the fall of the city but before.
(From: Francesco Gandolfo, "La basilica sistina: i mosaici della navata e dell'arco trionfale," in Chiese Monumentali d'Italia. Santa Maria Maggiore a Roma, edited by Carlo Pietrangeli [Florence 1988] p. 106 [translated by Bernard Frischer].)