When troops do things besides soldiering

Something I've been wondering in relation to the D@W: Campaigns rules:

If you're using your troops to work on construction projects, particularly something large-ish like circumvallations, when happens when the besieged enemy chooses that moment to sally out? Your heavy infantry find themselves holding nothing but shovels as the enemy cavalry hurtles towards them...

Is it assumed that all your troops will be ready to fight regardless of whether you're using them for other things.

A similar question applies to cavalry that give a scouting bonus when they're part of the army. If you're getting a large bonus from your cavalry on your scouting roll, would that mean that they're off trudging around looking for an enemy army, and thus if you were ambushed (d*mn useless scout cavalry), they wouldn't be avaialble to fight?

Or am I just overthinking all of this?

And come to think of it, assuming you're besieging a castle without circumvallation, what assumptions would you make when an enemy does sally out? Presumably your troops are spread all around the castle just out of bowshot range, so a fast-moving sortie would surely have the opportunity to smash into what could potentially be quite a thin line. That might be important if they're just trying to get a message or noble of import out of the siege.