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From Israel's Antiquities Authority:
To this day the reason the city walls were erected are still not known for certain. It is commonly believed, as E. Schiller and others have suggested, that the function of the city wall was not primarily military; rather it was meant to delimit the Holy City on the one hand, and protect its residents against raids by Bedouins and marauders on the other. This is attested to by among other things, the thickness of the walls, which at just 2.5 m wide is substantially less than that of the Hasmonean city walls that were 4-5 meters thick, and this is despite the fact that at the time the walls were being built cannons were already an essential weapon in warfare.
M. Ben-Dov proposes two other reasons for the construction of the city wall: 1) to protect against a renewed crusade by Carlos V, King of Spain, out of fear he wanted to conquer Jerusalem and 2) to reinforce Jerusalem from a religious standpoint so as to strengthen Muslim interests over the Christian interests in the city and thus "to win the encouragement of the vanquished Muslim population" (M. Ben-Dov 1983. P. 85). And in another place, "Jerusalem's fortifications were meant to promote the military goals of defending the city against powerful regular army forces and therefore their plan is excellent and many resources were invested in their construction. But no less important than this was the holiness of Jerusalem in the eyes of Islam⦠"(Ibid. p. 99)
References:
On E. Schiller - 1989 The Golan and the Battle of Gamla. In: M. Inbar, E. Schiller (eds.) Ramat Ha-Golan (Ariel 50β51):77β81 (Hebrew)
On M Ben-Dov 1983 - The Western Wall (Hakotel) (Adama Books, 1983)
Complete list of references from the site.