The Mishnah is explicit: Burial in towns surrounded by a wall is forbidden. Kelim 1.7. See Saul Lieberman, Tosefeth Rishonim, vol. 2, (Jerusalem, 1938), p. 135, and vol. 3, (Jerusalem, 1939), pp. 190–191. In the Dead Sea Scroll known as the Temple Scroll, the prohibition is implicit, but unmistakable. See Yigael Yadin, The Temple Scroll, (Jerusalem, 1983), vol. 2, col. 48, 11–14, p. 209; vol. 1, pp. 322–323. I am indebted to Professor Jacob Milgrom for these references.