This gruesome print memorializes the last two heads to be impaled on pikes over the Temple Bar, a stone archway that traditionally set off the western boundary of the city of London. Throughout the eighteenth century, the heads of traitors were routinely displayed on the roof of the Bar, which was called the Golgotha of Traitors.
Francis Townley and George Fletcher had been executed for their part in the 1745 rebellion, which was intended to place "Bonnie Prince Charlie" on the throne. The flag that the devil is waving over the archway was the banner of the rebellion, whose motto was "A Crown or a Grave." Under the devil's wing is the Stuart crest.
This is the scene as described in Old and New London (1878) via British History Online:
The heads of Fletcher and Townley were put on the Bar August 12, 1746. On August 15th Horace Walpole, writing to a friend, says he had just been roaming in the City, and "passed under the new heads on Temple Bar, where people make a trade of letting spy-glasses at a halfpenny a look." According to Mr. J. T. Smith, an old man living in 1825 remembered, the last heads on Temple Bar being visible through a telescope across the space between the Bar and Leicester Fields. . . .
The heads were left to "moulder in the sun and the rain" for twenty-six years
till the last day of March, 1772, when one of them (Townley or Fletcher) fell. The last stormy gust of March threw it down, and a short time after a strong wind blew down the other . . . .
According to Mr. Timbs, in his "London and Westminster," Mrs. Black, the wife of the editor of the Morning Chronicle, when asked if she remembered any heads on Temple Bar, used to reply, in her brusque, hearty way, "Boys, I recollect the scene well! I have seen on that Temple Bar, about which you ask, two human headsâ€”real headsâ€”traitors' headsâ€”spiked on iron poles. There were two; I saw one fall (March 31, 1772). . Women shrieked as it fell; men, as I have heard, shrieked. One woman near me fainted. Yes, boys, I recollect seeing human heads upon Temple Bar."
The print comes from Portaits, memoirs, and characters, of remarkable persons: from the revolution in 1688 to the end of the reign of George II collected from the most authentic accounts extant by James Caulfield' [which] is online in its entirety as page images among the Joseph McGarrity Collection at Villanova University Digital Library, Pennsylvania. Via BibliOdyssey