This source is a map of the towns in the Roman Empire. The distribution of the cities is very uneven, with a high density in Italy, Greece and Southern France, as well as along the coast. In the middle of the empire, including France, central Europe and central Britain, there are very few towns. There are no towns shown in Gaul, as it was not part of the Roman Empire.
Italy and Greece were established before the Roman empire, which explains the high density of towns. Italy is also the heart of the Roman empire, so a central population is needed to create a political and trade centre. The south of Spain is densely populated because the Romans conquered Hispania during the 3rd century BC, so when the Roman Empire began there were already Roman