Celestia's Smartypants

Celestia's special room is dark and narrow; but the scariest part about it, which Celestia is the proudest for forcing herself to install, is the single stair at the doorway.

Climacophobia

Realizing that she still suffers from age-old phobias; Luna recruits Twilight and Pinkie Pie to help Celestia overcome one of her deepest fears.

Stairs.