Thanks for the clarification. In my post preceding yours, I had also looked at Laurent de Premierfait and found where he did mention lightning - just not in the same place as Lydgate had put it, but in the moralization addressed to princes that followed (in Premierfait), still referring to the Tower of Babel. So the lightning is not Lydgate's invention, but, in relation to Boccaccio's, Premierfait's interpretation of Boccaccio's "hand of God." I also showed an illumination from ca. 1350 Naples in which the tower of Babel is shown with both lightning and figures falling from the tower. Comparing the Tower of Babel with the "as it were tongues of fire" descending at Pentecost that made the disciples speak in tongues was a popular medieval theme, with several examples in the link I provided, albeit not otherwise with fire or falling figures. But given the theme's popularity, it seems to me that Premierfait was probably drawing on an existing tradition, for the lightning and perhaps also the falling figures (although I do not see Lydgate specifying that they are falling from the tower, as opposed to falling from being hit by falling stones), as well as (clearly) Lydgate's illustrator. That there was a tradition in northern France for falling figures is suggested also by the Tower of Babel pictured in a Book of Hours acquired by the Duke of Bedford around 1420. So a 50% probability for falling figures as an original feature is reasonable enough.

What may well be Lydgate's illustrator's invention is putting a dragon, if that it is what it is, inside the tower entrance and on its outside while the building is still being constructed. Lydgate says nothing about dragons being in the Tower, or even being in the area then. And thanks, Ross, for identifying it as a dragon rather than a devil; I had not figured out what those odd black things on the tower were: they seemed too small to correspond to Lydgate's "gret dragouns," which he said infested the area, but said nothing about being in the Tower, much less when it was being constructed. However, if the people were giants and the tower very large, "gret dragouns," I suppose, could be like large lizards to us.

So Boccaccio adds serpents to the story, from an unknown source, perhaps Jewish, but referring to the place as it became after it was abandoned rather than in Nimrod's time. Premierfait preserves this detail. Lydgate adds "gret dragouns." The illustrator then puts them in or on the tower itself, perhaps reducing them in size, and makes them contemporary with Nimrod.

A dragon is not yet a devil. It becomes a devil only by association. There is the "great dragon" of Rev. 12. The concurrence of the two words suggests that perhaps Lydgate was making that association, and the red color the illustrator gives it fits that specified there. In any case, there is an association of dragons with devils: St. George's killing of the dragon has visual parallels to the archangel Michael's attack on Satan, for instance. In Latin, "draco" applied to both dragons and snakes, and we know the devil takes the form of a snake. One might even think, unless one noticed the other dragons in the illustration, that the Illustrator of Lydgate put a devil in the entrance. That this idea would get from London to Italy in time to make it into the "original" tarot, or have been part of a larger tradition as yet unknown to us, or have been invented independently in Italy then or earlier, without further evidence seems to me improbable but not impossible.