Because ~90% of the population is right-handed you want to keep the usurper's sword hand up against the wall as you retreat into your keep, while allowing yourself a fuller range of movement.
[Left: Castle of Emporeios, Greece; Right: Medieval Italian Fortress]
The Interior Design Gods conspired today to bring us two different projects on design blogs Dezeen and Contemporist that attempt roughly the same thing: adding a modern staircase into a historic stone castle. In the rural Italian village of Villa d'Adda, architect Gianluca Gelmini added his staircase as part of his renovation of a dilapidated 12th-century fortress. And on the Greek Island of Nisyros, Giorgos Tsironis and Greg Haji Joannides added their staircase to a 17th century building perched at the top of a hill. So, we put it to you, the reader. Which do you think is the more successful staircase?
[Medieval Italian Fortress]
[Castle of Emporeios, Nisyros, Greece]
Also at Curbed:
Starter Castles, Staircase Houses, and Dogs Named Tesla