Why did Dante place certain well-known historical figures in the ninth circle of Hell?
The ninth circle of Hell is made up of four different areas and its description extends from canto thirty-two to thirty-four. The ninth circle ends in the center of the Earth where Lucifer is stuck into ice. The most famous historical figures of these cantos are Count Ugolino, Judas Iscariot, Brutus and Cassius and they share Lucifer's fate of being stuck into the ice of the river Cocito. The fallen angel Lucifer or Dis (another name for Satan) is the symbol of the sinners imprisoned in this part of Dante's Inferno: just like Lucifer betrayed God, the historical figures that Dante put in this area committed some act of treason whether against relatives, guests, countries or benefactors/lords.As their heart was so cold and hard to lead them to betray their dear ones, so now they have to remain in the cold and hard ice (the logic of contrappasso or retailiation).
Count Ugolino became one of the better-known historical figures in the circle thanks to Dante's dramatic characterization of his fate. He is an example of treason against his country. Born into an influential Pisan family of Ghibellines (supporters of the Holy Roman Emperor), he switched sides aligning himself with the Guelphs (supporters of the Pope). Judas Iscariot, traitor of Jesus, is also imprisoned here and is being devoured by Satan. This is another example of contrappasso: as he worshipped evil in his life, he is destroyed by it in his afterlife. While Judas's guilt is agains the spiritual sphere of human life, Brutus and Cassius have sinned against the political sphere. As murderers of Caesar, they are responsible of killing the first great political leader who, to Dante, was able to keep together that vast Roman Empire that the writer still considered a political model for contemporary Italy.