The Tullianum was a prison in ancient Rome, located beneath the Comitium (large meeting space).
A circular chamber built of stone blocks, each layer projecting over the one beneath, the Tullianum was a conical structure described by Sallust (earliest known Roman historian with surviving works) as follows: "repulsive and terrible on account of neglect, dampness, and smell."
The only way in was via a hole in the ceiling. Jugurtha, King of Numidia, was tossed down the hole and later died of starvation.
The Tullianum wasn't a jail as we know it, but instead a place to store your captured enemy leaders until the time came to parade them around and let your citizens jeer at them. They lobbed a few other people down the hole too -- Christian martyrs, people who were going to testifying against defendants later on, that kind of thing.
Vercingetorix, who led the Gauls in revolt -- then surrendered to Caesar by taking off his armor and sitting at Caesar's feet -- spent a few years in the Tullianum before being brought up for the Walk of Shame ("Boo! Look at him! Boo! Hail Caesar!"). Then Vercingetorix went back down into the Tullianum and got strangled, which was a pretty standard method of execution.