I've been overly busy this past month but wanted to mention a new hypothesis that the bridge in the
Mona Lisa could be a reference to Rodomonte's Bridge, as featured in Italian epic poems by Boiardo (Orlando Innamorato, circa 1490) and Ariosto (Orlando Furioso, 1516). Leonardo's fondness for that genre is well-documented.
In addition, Leonardo and Ariosto shared a common patron in Isabella d'Este, of whom Leonardo made a very fine portrait around 1500 before starting the ML in 1503. The publication timeline of Ariosto's poem matches the final changes Leonardo made to the ML
before his death in 1519.
This is very speculative for now, just thought of today, but certainly interesting.