Final answer:
The Egyptian civilization was protected from invaders by geographical isolation, notably the Sinai Peninsula desert and the Nile River's cataracts. These barriers, along with the fertile Nile valley, contributed to Egypt's long period of stability and prosperity.
Step-by-step explanation:
The important natural barrier that kept Egyptian people safe from invaders was the geographical isolation provided chiefly by deserts and the Nile River cataracts. The expansive Sinai Peninsula desert created a formidable obstacle separating Egypt from Mesopotamia and the Near East. Likewise, the first cataract of the Nile served as a natural southern boundary, preventing easy passage by boats and limiting cultural influence and invasions from the south. The lack of advanced sailing technology also meant that there was minimal contact with other civilizations via the Mediterranean Sea.
Alongside these natural defenses, Egypt benefited enormously from its location on the fertile Nile River valley, which facilitated agriculture and the growth of a stable and prosperous civilization. The predictable flooding of the Nile ensured a steady supply of nutrient-rich soil, while the river itself acted as a natural highway for communication and transport. This combination of natural barriers and environmental advantages helped Ancient Egypt remain relatively insulated from foreign invasions and maintain continuity in its civilization for millennia.