Caesar visits the Gallic King Vercingetorix in a dungeon just prior to his triumph. Rome lacked this sort of dungeon, and foreign captives were held hostage by prominent, usually wealthy, families until the day they were to be shown at triumph. To display a captive king who was beaten and bent, as Vercingetorix is in the series, would do little to reinforce the glory of Rome, which was the main purpose of the triumph.
The way Caesar kills Vercingetorix is called "Garrote Vil", a Spanish's execution machine invented in the late 1400's.