I'm not sure that is a good analogy for the wall. Is it a different wall protecting thousands of cities or one of maybe ~10 walls (the main OS's) that is reused

I'm not sure that is a good analogy for the wall. Is it a different wall protecting thousands of cities or one of maybe ~10 walls (the main OS's) that is reused? Would it be that hard to build a few good walls? As you said though, there are always alternative ways to be attacked - robbed on the highway (man in the middle?) etc.