This page contains four variant views of the Aurelian Walls in Rome. The second sketch from the top clearly shows the outer side of the Porta Maggiore, also known as the Porta Praenestina, an ancient gate built to carry the waters of Rome’s two main aqueducts, the Aqua Claudius and the Aqua Anino Novus, over two of its major roads, the Via Labicana and Via Praenestina. The Emperor Aurelian later incorporated the arches of the gate into the line of his walls encircling the city, whilst in the middle ages, the Colonna family enclosed them within a fortified tower. See also folio 55 verso (D15400, CLXXXII 54a). The remaining three sketches on the page appear to show sections of wall and aqueduct seen near the gate. Cecilia Powell has suggested that they might have been executed from a moving vehicle.1
Powell 1984, p.472 note 11.