There were two major geographical features that helped protect Egypt from invaders. The first was the Mediterranean Sea to the north. This was a major obstacle to invaders because it meant that invaders would either need to use boats to transport attacking forces or take long and arduous routes through inhospitable terrain. Invading forces that did attempt naval attacks would then need to traverse the Nile Delta, a marshy and difficult barrier, before reaching the main parts of Egypt.
The second major barrier was the desert. Egypt was surrounded by desert to the east and west. Crossing the desert is difficult due to lack of water water and the dangers of extreme heat, making it almost impassible for large armies. Egypt really did not have powerful neighbors immediately close by, being separated from the major civilizations of Mesopotamia by sea and desert.