Well normally if you assault a castle you don't waste time and manpower with an encirclement. The attackers might have both the manpower and materials needed to quickly take the fortifications by storm, and if this is the attackers' preferred policy then it would be more beneficial to not encircle the place completely, because totally isolated defenders might be motivated by desperation to fight to the death while the presence of a hope for escape (however false) might cause demoralized defenders to rout and abandon the defenses prematurely--as in some dramatic instances during the Three Kingdoms period in Chinese history.

Though since they don't want any survivors they might actually encircle anyway. Ya that makes sense, so Nargesh's sally can be a diversion, after all it would be a very foolish defender indeed who would let the besieger work unhindered along the siege-lines; as long as there are warriors /blades to spare, any good defending commander would send small parties with missions to wreck the attackers' siege engines, disturb their camps, spoil or steal their supplies, harry their trench-digging or fort-building parties, and mess up with their preparations for escalade or similar endeavors.

On that note then, who want's to be part of the sally to disrupt the enemies siege weapons and preparations? Heck I imagine two different sally teams could work best to cause some confusion and avoid the enemy focusing on one. As a side note, Nargesh can carry at most one other person. :P

 Seeing as our enemy is underestimating us good chance they won't expect a sortie.