What made the fortresses built by Ancient Egyptians so impenetrable?

Buhen (Ancient Greek: Βοὥν Bohón)[1] was an ancient Egyptian settlement situated on the West bank of the Nile below (to the North of) the Second Cataract in what is now Northern State, Sudan. It is now submerged in Lake Nubia, Sudan. On the East bank, across the river, there was another ancient settlement, where the town of Wadi Halfa now stands. The earliest mention of Buhen comes from stelae dating to the reign of Senusret I.[2] Buhen is also the earliest known Egyptian settlement in the land of Nubia.[3]

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