Hellenistic period
The vast Seleucid Empire was, like Egypt, mostly dominated by a Greco-Macedonian political elite. The Greek population of the cities who formed the dominant elite were reinforced by emigration from Greece. These cities included newly founded colonies such as Antioch, the other cities of the Syrian tetrapolis, Seleucia (north of Babylon) and Dura-Europos on the Euphrates. These cities retained traditional Greek city state institutions such as assemblies, councils and elected magistrates, but this was a facade for they were always controlled by the royal Seleucid officials. Apart from these cities, there were also a large number of Seleucid garrisons (choria), military colonies (katoikiai) and Greek villages (komai) which the Seleucids planted throughout the empire to cement their rule. This 'Greco-Macedonian' population (which also included the sons of settlers who had married local women) could make up a phalanx of 35,000 men (out of a total Seleucid army of 80,000) during the reign of Antiochos III. The rest of the army was made up of native troops. Antiochus III the great conducted several vigorous campaigns to retake all the lost provinces of the empire since the death of Seleucus I. After being defeated by Ptolemy IV's forces at Raphia (217), Antiochus III led a long campaign to the east to subdue the far eastern breakaway provinces (212-205) including Bactria, Parthia, Ariana, Sogdiana, Gedrosia and Drangiana. He was successful, bringing back most of these provinces into at least nominal vassalage and receiving tribute from their rulers. After the death of Ptolemy IV (204), Antiochus took advantage of the weakness of Egypt to conquer Coele-Syria in the fifth Syrian war (202-195). He then began expanding his influence into Pergamene territory in Asia and crossed into Europe, fortifying Lysimachia on the hellespont, but his expansion into Anatolia and Greece was abruptly halted after a decisive defeat at the Battle of Magnesia (190 BCE). In the Treaty of Apamea which ended the war, Antiochus lost all of his territories in Anatolia west of the Taurus and was forced to pay a large indemnity of 15,000 talents.

Q: What are Seleucid military colonies called?
katoikiai