The Roman Empire reached its largest territorial expanse during the reign of Trajan (AD 98–117), encompassing an area of about 5 million sq km (1.93 million sq m).
The maps below show The Roman Empire at its territorial height.
The topography of the Roman Empire
The Roman army mainly was infantry based, which was great in hilly or mountainous terrain but at a critical disadvantage to cavalry on the open flats. As a result, the Roman Empire’s frontiers mainly situated on one of the following:
- Coast (the entire Mediterranean basin, Iberia, most of northern and western Europe)
- Mountains (Balkans, Alps, north-western Africa, Anatolia in modern Turkey, Middle East)
- Desert (northeastern Africa and the Arabian peninsula)
Any of the above is an efficient barrier to the tactical movement and logistical mobility and supply of large cavalry-based armies, the curse of the Roman army during its existence, and the reason for some of its most dramatic losses, from Cannae to Carrhae.
Behind tactical considerations, swaths of rough territory also give a natural geographic constraint to population density overall. On such edges, the Romans were less likely to have a lot of the enemy population dwelling close to the border, to begin with, suggesting a thinly-stretched garrison could protect a large part of the border as far as local border clashes went, while also having more time to prepare a response to any large attacks before they could breach the guarded perimeter.
Below is the fantastic relief map of the Roman Empire created by @mappdoutofficial.