Commentary Purg XII 61-63

The final example serves as a sort of summary for the entire acrostic.  For the medieval tradition that Troy was indeed prideful see Scott (Scot.2001.1), pp. 182-83.  Dante's phrasing probably reflects Aeneid. III.2-3, 'superbum / Ilium,' with the phrase to be read in a moralizing ('proud Troy') rather than an architectural ('lofty-towered Troy') manner.  See C.Inf.I.75, discussing an earlier incidence of the same phenomenon.