Many of these ‘new’ cities founded by the Romans were not new at all. They simply renamed and rededicated existing cities with religious rites and / or legal designations. Sometimes this involved the movement of people, and sometimes it did not. Some colonies were founded by new settlers – particularly early in Roman history – and sometimes an existing settlement was simply made a colony or a ‘new’ city to reward their inhabitants. The granting of Roman citizenship to the local and municipal elites often went with this granting of the status of colony.

You mentioned the modern city of Istanbul. This was ‘founded’ by Constantine but in actuality the city had existed as Byzantium for about 900 years prior to its ‘foundation’ by Constantine as New Rome.

Sometimes the Romans did build new cities, and we do have a manual of sorts of what to do: the fourth chapter of Vitruvius. He stresses the local geography and the health of a site were important. He suggests it be built on higher ground, ‘neither misty nor frosty, and in a climate neither hot nor cold, but temperate…’

Marshes were particularly unhealthy and should be avoided. Vitruvius thinks there was bad air around marshes, but most likely he was referring to malaria spread by mosquitos. So he was right that cities in marshes could be unhealthy for the inhabitants. If a city had to be built in marshy grounds Vitruvius recommends ditches be dug to drain them or to allow them to be flushed with sea water.

Vitruvius also stresses the importance of good food and water being available in the near area. Additionally, depending upon the immediate locale, the new site might need to be defensible and need city walls or other fortifications.

Finally, we can’t forget how important religious ceremonies were for the Romans. Vitruvius insists that proper rites should be followed, such as allowing cattle to feed on the proposed site and then sacrificed so their livers could be examined. If the livers were abnormal, that meant that the food and water in that place was unhealthy and should be avoided.

Vitruvius doesn’t mention this, but we know from other sources that the sacred boundary of a city was extremely important. The ritual performed by Romulus involved ploughing the line of the pomerium. We also know that sacred processions followed the boundary of the ‘new’ city. Constantine probably did something of this sort for Constantinople, but there is some controversy if it was a pagan ceremony, Christian ceremony, or some hybrid.