Administrative, legal, fiscal, and military power in the Roman Empire came from the Roman emperors and the centres of governance were at their locations. Throughout the history of the empire multiple cities were used by the emperors as their place of residence, for minting coinage, or where they issued major legislation.
Rome was the centre until the Crisis of the Third Century and Tetrarchy, but returned to importance in the 5th century. Antioch, Nicomedia, Thessalonica, Sirmium, Trier, and Mediolanum were used as residences by multiple emperors in the 3rd and 4th centuries. Constantinople was dedicated by Constantine the Great in 330 AD and became the permanent seat of the emperor in the east starting with Theodosius I. After 312 AD no other emperor in that century resided in Rome and Ravenna was used as a residence by Honorius and Valentinian III, but the western emperors used both cities before the disposition of Romulus Augustulus.
Governance The system of government present in the Roman Empire resulted in all administrative, legal, fiscal, and military power being vested in the Roman emperor and thus the centre of governance was at his location. The state was centralised at the location of the emperor rather than a specific city. The court moved with the emperor. Augustus (r. 27 BC – 14 AD) acquired the legal powers of the Senate and magistrates for the emperor during his reign. Caligula (r. 37 – 41) was the first emperor to acquire all of his powers at once rather than over the course of his reign. The Tetrarchy divided the empire so that it was administered by two augusti and two caesars. Despite governance being divided between four people all laws were enacted in the names of all four.
Rome was host to the Senate and the praefectus urbi. Cassius Dio wrote that public politics ceased to exist after Augustus and that important issues were no longer voted on, but instead discussed in private with the emperor. Foreign and domestic embassies came to the Senate until the 100s AD. Copies of treaties were placed into the Temple of Jupiter Optimus Maximus. A new senate was created for Constantinople and these senators were given equal rank to those in Rome by Constantius II in the 350s. A prefect for Constantinople was established in 359, and a later law restricted the prefect of Rome to hearing appeals from the Italian provinces while the prefect of Constantinople could hear appeals from Thrace and Asia Minor.
Vespasian (r. 69 – 79) and Hadrian (r. 117 – 138) oversaw legal cases in public in Rome while Trajan (r. 98 – 117) oversaw conducts his in private in Civitavecchia. Diocletian (r. 284 – 305) issued major edicts while in Nicomedia, Antioch, and Alexandria. A meeting between Constantine the Great (r. 306 – 337) and Licinius (r. 308 – 324) in Mediolanum produced the Edict of Milan. Theodosius I enacted laws in Constantinople, Selymbria, Heraclea, Adrianople, Beroe, Cosintus, Thessalonica, Stobi, Scupi, Ulpiana, Sirmium, Aquileia, Concordia, Vicenza, Verona, Mediolanum, Forum Flaminii, and Rome. Arcadius issued laws in Constantinople, Nicomedia, Nicaea, Mnizus, and Ankara. Theodosius II issued 95% of his known laws in Constantinople while the remainder were issued in Nicomedia, Selymbria, Topirus, Aphrodisias, Apamea Myrlea, and Cyzicus.
Coinage
The mint in Alexandria established by the Ptolemaic Kingdom was maintained by the Roman Empire; it maintained its own separate coinage system of tetradrachm before being made uniform with the Roman coins in 296 AD. The client rulers of the Bosporan Kingdom were the only non-Romans to produce aureus.
The senatorial mint in Rome stopped producing bronze coins in 82 BC and gold and silver coins in 36 BC. All coinage was struck in the provinces by either Mark Antony or Augustus from 36 to 31 BC. From 31 to 14 BC coinage was struck in the provinces by Augustus. The senatorial mint of Rome was reopened by Augustus in 23 BC. Around 14 BC the many provincial mints of the empire were closed and a large mint was created in Lugdunum. All gold and silver coins in the empire were struck in Lugdunum under Augustus and Tiberius (r. 14 – 37). A mint was reestablished in Rome by the time of Nero (r. 54 – 68).
Coinage was struck by Galba (r. 68 – 69) in Gaul and Spain, Otho (r. 69) in Rome, Vitellius (r. 69) in Rome, Lugdunum, and Tarraco, and Vespasian across the empire after initially using eastern mints. After Vespasian established control over the empire he closed all of these new mints except for the one in Antioch. Lugdunum became the main site of striking coinage in 74 AD and Rome became the main centre of striking coinage further into Vespasian's reign. This system was in place until Gordian III (r. 238 – 244) opened a mint in Viminacium with the exception of local coinage used for major military campaigns led by the emperor, the travels of Hadrian, and the Year of the Five Emperors.
At the beginning of the 3rd century most of the coinage was produced in Rome and Antioch, but became decentralised during this century. The prevalence of Antioch's mint rose starting under Septimius Severus (r. 193 – 211) due to civil war and conflicts with the Persians. The mint in Viminacium was moved to Mediolanum by Decius (r. 249 – 251) due to threats from the Goths, but was returned to Viminacium by the time of Valerian (r. 253 – 260). Gallienus (r. 253 – 268) established a mint in Mediolanum after losing access to the mints in Gaul now controlled by the Gallic Empire. Antioch, despite being controlled by the Palmyrene Empire, was nominally loyal to the Romans and continue to mint coinage featuring the Roman emperor. Roman control over the mints of Gaul was reestablished by Aurelian (r. 270 – 275). Under Aurelian the three largest mints of the empire were Rome, Siscia, and Mediolanum. After launching a revolt Carausius (r. 286 – 293) established mints in Britain.
When Diocletian became emperor there were seven mints in the empire. After Diocletian's reforms there were fourteen main mints in the empire, eight in the west and six in the east. Constantine used Sirmium as his political and military base from 319 AD to 324 AD, and a mint operated there from 320 to 325/326. The centre of coinage striking shifted to the east with the establishment of a mint in Constantinople in 326 AD and the closure of the mints in Carthage, London, and Ticinum.
Gordian I and Gordian II (r. 238) never visited Rome during their reign of three weeks, but their coinage was produced there. Coinage was struck in Rome for Anthemius (r. 467 – 472), in Ravenna for Joannes (r. 423 – 425), both Rome and Ravenna for Julius Nepos (r. 474 – 480), and in both Ravenna and Mediolanum for Glycerius (r. 473 – 474).
Imperial residence Principate After Augustus's military campaigns in Spain from 26 to 25 BC, the emperor mainly delegated military leadership to others. Emperors started to take more personal control of the military by the time of Domitian (r. 81 – 96). Augustus conducted three year-long trips to the provinces followed by two years in Rome for fifteen years. Suetonius wrote that Augustus personally went everywhere held by Rome except for Africa and Sardinia.
Tiberius was frequently absent from Rome and this led to praetorian prefect Sejanus acquiring a significant amount of power. Tiberius was away from Rome for 20 months from 21 to 22 AD and withdrew to Capri in 26 AD. Tiberius did not cease to control the empire after leaving Rome, but Sejanus controlled the flow of information between Tiberius and Rome and Rome remained the centre of major governance.
Galba was acclaimed caesar by his soldiers in Carthago Nova, but he rejected this title and instead titled himself "Legate of the Roman Senate and People". Galba was based in Clunia during his rebellion against Nero and was there when he learned of Nero's death. Vitellius was hailed as emperor in Cologne, Mainz, Neuss, and Xanten. Otho seized power after assassinating Galba and receiving recognition from the Senate. Vitellius was in Lyons when he learned of Otho's death after the Battle of Bedriacum and spent months in northern Italy before arriving in Rome, where he was recognised by the Senate. Vespasian rebelled with the support of the eastern portion of the empire and was acclaimed emperor by his soldiers in Caesarea Maritima.
Nerva (r. 96 – 98) adopted Trajan in 97 AD, but Trajan did not arrive in Rome until 18 months into his reign in 99 AD; Trajan was in Colonia Claudia Ara Agrippinensium when Nerva died. Trajan spent almost half of his reign outside of Rome and the longest period was the six years when he fought the Dacian Wars and Parthian campaign. Hadrian conducted multiple years long tours of the empire and was outside of Italy for half of his reign. Antoninus Pius (r. 138 – 161) is known to have only left Italy for one year in his life and it occurred before his time as emperor when he was proconsul of Asia.
Lucius Verus (r. 161 – 169) personally led a war against Parthia from 161 to 166 AD while Marcus Aurelius remained in Rome (r. 161 – 180). Both emperors left Rome to lead the army in the Marcomannic Wars in 168 AD, and this was the first time that Marcus Aurelius left Italy. However, Marcus returned to Rome after the death of Lucius Verus in 169 AD. Marcus returned to the campaign and used Sirmium and Carnuntum as his headquarters. After a tour of the east in response to Avidius Cassius's uprising Marcus returned to Rome in 176 AD. He remained in Rome for 18 months before returning to fight in the Marcomannic Wars. He died in either Sirmium or Vindobona in 180 AD.
Commodus (r. 177 – 192) was uninterested in governmental affairs and these were instead handled by Sextus Tigidius Perennis and Marcus Aurelius Cleander. Pertinax and Didius Julianus became emperors in Rome during the Year of the Five Emperors in 193 while Septimius Severus was acclaimed in Carnuntum, Pescennius Niger in Antioch, and Clodius Albinus in Britannia.
Caracalla (r. 198 – 217) spent most of his reign outside of Rome on military campaigns or touring the provinces. At the time of Caracalla's death he was located near Edessa and his imperial retinue was based in Antioch.
Crisis of the 3rd Century Macrinus (r. 217–218) never returned to Rome as emperor and remained based in Antioch for the duration of his reign.
Maximinus Thrax (r. 235 – 238) was proclaimed emperor by soldiers stationed along the Rhine and remained there before being killed while trying to capture Aquileia. The Senate declared Gordian I, Gordian II, Pupienus, and Balbinus as emperor in opposition to Thrax. Gordian III travelled with his entire court to the east for a military campaign against the Sasanians, but died and was succeeded by Philip the Arab (r. 244 – 249). Philip went to Rome to secure his position.
Multiple generals were declared emperor by their soldiers along the front during the crisis of the 3rd century, such as Decius, Trebonianus Gallus, Aemilianus, Valerian, Claudius Gothicus, Quintillus, Aurelian, and Diocletian.
Valerian was captured by the Sasanian in 260 and this left Macrianus Major in charge of the war treasury in Samosata. Macrianus relocated his base to Emesa and then Antioch before defeating the Sasanians. He and his sons Macrianus Minor and Quietus were recognised as emperors in Anatolia and Egypt, but were defeated by Gallienus in 261. Odaenathus was given the title "Marshal of the East", which gave him military and civil powers in Syria, for helping to defeat Macrianus. After the death of Odaenathus the east came under the control of Zenobia.
Carus (r. 282 – 283) never came to Rome during his reign while Carinus (r. 283 – 285) visited twice. Numerian (r. 283 – 284) and Carinus divided and governed the empire into eastern and western sections.
Tetrarchy
None of the augusti or caesars of the Tetrarchy ruled from Rome and Diocletian first entered Rome in 303 AD to celebrate the 20th anniversary of his reign.