MCRP 12-10B.1, page 79

reconnoitered and marked underground systems trenches and sewage systems concealment offered by smoke and darkness to cross open areas. (d) O: Obstacles. A city is an obstacle because it canalizes and impedes an attack. Avenues of approach should be blocked by obstacles and covered by fire. (See Paragraphs 3404.d., 3404.f., 3405.e., and Appendix F.) (e) A: Avenues of Approach. Avenues of approach (surface and subsurface) should be denied to the enemy in keeping with the overall defensive plan (Figure 3-4). (2) Weather. The effects of weather as discussed in Chapter 2, Section II, pertain equally to defensive operations. D. Troops and Support Available. The defender of a built-up area has a terrain advantage and can resist the attacker with much smaller forces. The defender stands a greater chance of success when he/she fights with combined arms and task organizes his/her forces. The discussion of troops and support available in Chapter 2, Section II, also pertains to defensive operations. Figure 3-4. Denial of Avenues of Approach MCWP 3-35 .3 3-8