sentences:  Historian Adrian Goldsworthy argues that the Romans had reached the natural limits which their military traditions afforded them conquest over and that beyond the borders of the early-to-mid Empire lay peoples whose military traditions made them militarily unconquerable, despite many Roman battle victories.In particular, Goldsworthy argues that the cavalry-based warfare of the Parthians, Sarmatians and Persians presented a major challenge to the expansion of Rome's infantry-based armies.The borders of the Roman Empire, which fluctuated throughout the empire's history, were a combination of natural frontiers (the Rhine and Danube rivers to the north and east, the Atlantic to the west, and deserts to the south) and man-made fortifications which separated the lands of the empire from the "barbarian" lands beyond.