Extract the answer to the question from the following context. Question: What in Africa kept Rome from expanding its empire further? Context: The Jugurthine War of 111104 BC was fought between Rome and Jugurtha of the North African kingdom of Numidia. It constituted the final Roman pacification of Northern Africa, after which Rome largely ceased expansion on the continent after reaching natural barriers of desert and mountain. Following Jugurtha's usurpation of the throne of Numidia, a loyal ally of Rome since the Punic Wars, Rome felt compelled to intervene. Jugurtha impudently bribed the Romans into accepting his usurpation. Jugurtha was finally captured not in battle but by treachery.
natural barriers of desert and mountain