The thing about Orcs occupying former Elven fortresses is that Orcs are predominantly left-handed. This puts them at a disadvantage when defending: staircases, doors, and built-in furnishings were designed to free up the right hand of defenders, while obstructing the right side of attackers.

This is partly why Sam could so successfully storm Cirith Ungol. Having your dominant hand against the wall on a staircase is a huge impediment.

[File this under "shit Dan made up"]