Romans did not have prisons like the ones in the modern world.

Accused wealthy citizens were simply kept under house arrest, provided they behaved, until a trial could take place. Occasionally the accused might be detained to await trial, but usually those awaiting trial were encouraged to go into voluntary exile. Those awaiting trial were called "carcer" or "publica vincula."

At the foot of the Capitoline Hill, between the Curia and the Temple of Concord stood the Carcer, the only state prison of ancient Rome. It is sometimes called the Mamertine Prison. The carcer was the upper section in which prisoners could be held awaiting sentence.

Mamertine prison. Entrance

The underground area of the prison was called the Tullianum because it housed water springs. Executions occurred here. According to tradition, St. Peter and Paul were confined here during the reign of Nero and reputedly St. Peter called up the waters of the spring to baptize his jailers.

The Epistles to the Colossians, the Ephesians, and Philemon were despatched together and by the same messenger, Tychicus.

Timothy was born in Lycaonia in Asia Minor. His mother was a Jew and his father was a Gentile. When Paul came to preach in Lycaonia, Timothy, his mother and his grandmother all became Christians. Several years later, Paul went back to found Timothy grown up. Paul invited him to join him in preaching the Gospel. Timothy was the great apostle's beloved disciple, like a son to him. He went everywhere with Paul until he became bishop of Ephesus. Then Timothy stayed there to shepherd his people. As St. Paul, Timothy, too, died a martyr.

An Asiatic Christian, a "faithful minister in the Lord" (Eph. 6:21, 22), who, with Trophimus, accompanied Paul on a part of his journey from Macedonia to Jerusalem (Acts 20:4). He is alluded to also in Col. 4:7, Titus 3:12, and 2 Tim. 4:12 as having been with Paul at Rome, whence he sent him to Ephesus, probably for the purpose of building up and encouraging the church there.

There is no doubt that Paul's trial terminated in a sentence of acquittal, for the report of the Governor Festus was certainly favorable as well as that of the centurion.

There is no doubt that Paul's trial terminated in a sentence of acquittal, for The Jews seem to have abandoned their charge since their co-religionists in Rome were not informed of it (Acts, 28: 21).

(Act 28:21) And they said unto him, We neither received letters out of Judaea concerning thee, neither any of the brethren that came shewed or spake any harm of thee.

The course of the proceedings led Paul to hope for a release, of which he sometimes speaks as of a certainty

Phi 1:25 Convinced of this, I know that I shall remain and continue with you all, for your progress and joy in the faith.

Phi 2:24 and I trust in the Lord that shortly I myself shall come also.

(Phm 1:22) At the same time, prepare a guest room for me, for I am hoping through your prayers to be granted to you.

Scholars have been trying to reconstruct what happened after the release. Many have been able to point out possible routes of Paul after the freedom till his rearrest and martyrdom.

1. Paul expressed intention to travel westward from Rome to Spain (Rom 15:24) as well as eastward from Rome to Macedonia and Asia Minor (Phil 2:24; Philem 22). Evidence suggests he carried out both journeys:

a. Paul evangelized the island of Crete in the east (Titus 2:5); this mission would not have been possible to fit into Paul’s earlier travel routes.

b. Clement of Rome - Paul carried the Gospel to limits of the west

c. Tradition holds Paul was later rearrested and martyred (beheaded) in Rome

Eusebius tells that Paul arrived "a second time in this town" of Rome and that he suffered there a martyr's death. And Christians might be "proud that such a man" persecuted them: "for he who knows Nero, understands that he would not have condemned this teaching unless it had been something extremely good."

Commentary on the Bible , Adam Clarke says:

"Concerning the time, place, and manner of his death, we have little certainty. It is commonly believed that, when a general persecution was raised against the Christians by Nero, about A.D. 64, under pretence that they had set Rome on fire, both St. Paul and St. Peter then sealed the truth with their blood; the latter being crucified with his head downward; the former being beheaded, either in A.D. 64 or 65, and buried in the Via Ostiensis. EUSEBIUS, Hist, Eccles. lib. ii. cap. 25, intimates that the tombs of these two apostles, with their inscriptions, were extant in his time; and quotes as his authority a holy man of the name of Caius, who wrote against the sect of the Cataphrygians, who has asserted this, as from his personal knowledge. See Eusebius, by Reading, vol. i. p. 83; and see Dr. Lardner, in his life of this apostle, who examines this account with his usual perspicuity and candor.

"Other writers have been more particular concerning his death: they say that it was not by the command of Nero that he was martyred, but by that of the prefects of the city, Nero being then absent; that he was beheaded at Aquae Salviae, about three miles from Rome, on Feb. 22; that he could not be crucified, as Peter was, because he was a freeman of the city of Rome. But there is great uncertainty on these subjects, so that we cannot positively rely on any account that even the ancients have transmitted to us concerning the death of this apostle; and much less on the accounts given by the moderns; and least of all on those which are to be found in the Martyrologists. Whether Paul ever returned after this to Rome has not yet been satisfactorily proved. It is probable that he did, and suffered death there, as stated above; but still we have no certainty" (Commentary on the Bible by Adam Clarke, commenting on Acts 28:31).

The International Standard Bible Encyclopedia says:

"When Paul writes again to Timothy he has had a winter in prison, and has suffered greatly from the cold and does not wish to spend another winter in the Mamertine (probably) prison (2Timothy 4:13, 21). We do not know what the charges now are. They may have been connected with the burning of Rome. There were plenty of informers eager to win favor with Nero. Proof was not now necessary. Christianity is no longer a religion under the shelter of Judaism. It is now a crime to be a Christian. It is dangerous to be seen with Paul now, and he feels the desertion keenly (2Timothy 1:15ff; 4:10). Only Luke, the beloved physician, is with Paul (2Timothy 4:11), and such faithful ones as live in Rome still in hiding (2Timothy 4:21).

"Paul hopes that Timothy may come and bring Mark also ( 2Timothy 4:11). Apparently Timothy did come and was put into prison (Hebrews 13:23). Paul is not afraid. He knows that he will die. He has escaped the mouth of the lion (2Timothy 4:17), but he will die (2Timothy 4:18). The Lord Jesus stood by him, perhaps in visible presence (2Timothy 4:17). The tradition is, for now Paul fails us, that Paul, as a Roman citizen, was beheaded on the Ostian Road just outside of Rome. Nero died June, 68 AD, so that Paul was executed before that date, perhaps in the late spring of that year (or 67). Perhaps Luke and Timothy were with him. It is fitting, as Findlay suggests, to let Paul's words in 2Timothy 4:6-8 serve for his own epitaph. He was ready to go to be with Jesus, as he had long wished to be (Philippians 1:23)"

Since Paul was a Roman citizen, he could not be executed within the city of Rome, nor could he be crucified. He was taken outside the city of Rome and put to death with a sword. St. Paul Basilica is built over his tomb. He was beheaded between 66-68 AD at Aquae Salviae, which is now known as Tre Fontane.

Legend says that his head bounced three times, and a fountain sprung up at each stop – hence the name Tre Fontane, or Three Fountains. His body was taken about two miles away to be buried in land owned by a friend, where the Basilica of St. Paul Outside the Walls was later built.

The story of St. Paul's martyrdom is told in Jacobus de Voragine's Golden Legend (Legenda Aurea) as follows:

Condemned to death by Emperor Nero, Paul was taken to the place of his execution outside the Ostia Gate in Rome, hence the appearance of Pyramid of Cestius in the background. On his way he not only converted three of the Roman soldiers who were his captors (here represented by the soldiers in armor at the bottom and to the left of the scene) but also drew the sympathy of a Roman matron named Plautilla, or Lemobia, who was a Christian. She asked him to pray for her and he responded by asking her for her veil with which to cover his eyes, assuring her that she could have it back when the grisly execution was over. The executioners mocked her, saying, "How canst thou give this precious object to such an imposter."