The communists party of the Czechoslovakia
The eastern and western deserts prevented invaders from the east and west. The deserts were a hard climate to travel through. Therefore, nobody could walk across to conquer Egypt. Furthermore, the cataracts in the Nile to the south protected the Egyptians from lands below them. So the answer would be that The Nile protected the Egyptians from invasion.
It was the Mamluks. A Mamluk was a soldier who converted to Islam, and over the time that the military used, they became stronger and more powerful military. This was the only way and the only war that the Mongols lost.