According to the rules, they can. That isn't the problem.
The problem is that from the front line, they have to use their defense
rating. So if I want to have a _really_ secure castle, I need to allocate
two nobles to it round-the-clock, one for a pikewall (or knights/elite
guards) and the other for missile troppos somewhere behind.
Tom, who suffers from a cyclic embarassment of excess crossbowmen