License, verbatimPD
The National Gallery of Art: open access image with no usage restrictions under the NGA's open access policy (openaccess=1 in the Gallery's published_images dataset).
Carlo Baldassare Simelli. Fortificazioni dei bassi tempi sopra il Ponte Nomentano (Fortifications of the Ponte Nomentano), 1830s-1840s. National Gallery of Art, Washington.