Would a left-handed attacker fare better on a spiral stair?

Would a left-handed attacker have an advantage on a castle spiral stair?

Yes, a left-handed attacker would have more room for the weapon arm, although shield movement could still be difficult.

A left-handed attacker would have an advantage climbing a conventional spiral stair because the weapon arm would have more room. Shield movement could still be awkward. Handedness changes the balance of these constraints without removing them.