Answer:
The location, and the buildings.
Explanation:
the ancient Egyptians living places were located in the middle of the deserts and they were adapted to the living habits there, for invaders to invade ancient Egypt,
1. they got to be a desert experts
2. they should know the coordination's of the places in the middle of the desert.
However, the buildings made it even harder for invaders to invade ancient Egypt due to its unique building structure for example the pyramids in the ancient days used to have guards all around, on top, of it where they had the height and the sight of every movement. :)