If they breach the walls and actually enter the interior of the castle, they have already lost and no staircase design can save them.
If they breach the walls and actually enter the interior of the castle, they have already lost and no staircase design can save them.
Castles weren't single layer defenses that stopped at the walls, they were definitely designed to continue the fight and protect the nobles even if the enemy did get through the walls. Given that it's not so out of line to think the interior architecture might have been designed to increase defender's advantages.
If there were no help from outside, next would be keep wall, or most probably lack of food and water for defenders.
Also I'll admit the first couple arguments in TFA felt weak. I wouldn't be surprised if that colored peoples readings of the rest, or if they jumped to come comment before finishing the rest.
That's assuming that the castle has fallen by assault. If the attacker was an assassin who had gained access to the castle through subterfuge or a disgruntled worker then interior defences may have conceivably been useful.