What geographic feature allowed the Roman Empire to spread?

There were geographic features which limited expansion of the Roman Empire only in two areas. One was the Sahara Desert, which meant that the Romans stayed on the coastal areas of Algeria and Libya. The other was the highlands of Scotland. The favoured the fierce resistance to Roman expansion by the Picts. Given that the mountains made it difficult to subdue them and that Scotland was a peripheral area with little economic benefits, the Romans withdrew and built two walls as protection from raids by the Picts: Hadrian's Wall and the Antonine Wall.

Apart from the above, Geographical obstacles, such as mountains and wide rivers, were no deterrent for Roman expansion.