Seleucia on the Tigris: one of the residences of the Seleucid Empire, modern Tell Umar, on the west bank of the Tigris. The city is not to be confused with the royal residence named Seleucia in Pieria, or the little known Seleucia on the Euphrates.
It was explictly designated as the capital of the empire (āl šarrūti in cuneiform sources, "city of kingship"). The city was opposite the ancient city of Opis at the confluence of the river Tigris and the Royal Canal, which connected the new city to the Euphrates. The ruins have been identified at Tell Umar, about 30 km south of Baghdad, and 60 north of Babylon, which appears to have lost some of its citizens to the new city.note[Pausanias, Guide to Greece 1.16.3; Antiochus and the Sin Temple Chronicle.]
Excavations have shown that the city was built according to a gridiron plan. As it was a Seleucid city, there will have been the usual, straight main street, perhaps decorated with colonnades. The course of the walls will have been determined by the shape of the site - Pliny the Elder compares the plan of the walls of Seleucia "is like an eagle spreading its wings".note[Pliny the Elder, Natural History 6.122.] A canal separated the official residence from the living quarters, where people were - according to a Babylonian custom - buried inside their homes. The agora has not yet been identified; the theater may have been on the southern edge of the city.
According to Pliny, the city had 600,000 inhabitants.note[Pliny the Elder, Natural History 6.122.] This may, for once, be not exaggerated: Babylonia is extremely fertile and it was easy to bring food to the city. Baghdad was to have millions of inhabitants, which had to be fed from the same resources, using the same technology.
The city was an important center of Greek civilization. Strabo mentions an astronomer named Seleucus, and adds that he was educated in the Babylonian traditions of stellar science (a "Chaldaean");note[Strabo, Geography 16.1.6.] he also mentions a Stoic philosopher named Diogenes,note[Stabo, Geography 16.1.16.] who was - interestingly - of native Babylonian descent.note[Plutarch, Fortune of the Alexander 328d.] According to Plutarch, the rhetor Amphicrates believed he could find a worthy position in Seleucia, was offered one, but in the end refused it, comparing the city to a stewpan.note[Plutarch, Lucullus 22.5.] Perhaps this is a reference to the climate.
The city was conquered in 141 BCE by the Parthian king Mithradates I the Great (171-138, who continued to use the city as one of the mints. The new rulers added a new city, Ctesiphon, on the east bank. Pliny claims that in his age, the city still had some Macedonian customs.note[Pliny the Elder, Natural History 6.122.] Other ethnic groups were the Greeks, the Babylonians, and a large group of Jews, which fled to the city after 35 CE, and was massacred.note[Flavius Josephus, Jewish Antiquities 18.311.]
The twin cities were captured several times by the Romans. The emperor Trajan took them in 116, general Avidius Cassius during Lucius Verus' Parthian campaign.note[Cassius Dio, Roman History 68.30.2 and 71.2.3.] This may have been the end of Seleucia, and it is slightly ironical that the Roman general, Avidius Cassius, was a descendant of its founder, Seleucus Nicator.
In the fifth century, Seleucia was an important center of Nestorian Christianity; the Nestorian teachings were defined during the ecclesiastical council of 468.