Final answer:
Difficult geography and natural defenses such as deserts, rivers, and man-made fortifications like walls and ditches, protected settlements in the Fertile Crescent from attack.
Step-by-step explanation:
The physical feature that made it difficult for outsiders to attack a settlement in the Fertile Crescent was the region's challenging geography and natural defenses. One example is the desert of the Sinai Peninsula, which acted as a barrier between Egypt and the civilizations of Mesopotamia and Anatolia. This roughly 120 miles of desert was a formidable obstacle to potential invaders, limiting cross-regional contact and incursions. Similarly, Jericho, another settlement in the broader region of the Fertile Crescent, leveraged natural fortifications like a large ditch and a thick stone wall that encircled the settlement for defense.
Additionally, the unpredictable flooding of the Tigris and Euphrates rivers could create natural barriers. The natural defenses contributed to the growth and stability of cities such as Ur, Babylon, and Nineveh which developed along these riverbanks.