The Sorcerer's Stone (J.K. Rowling 1997, Ch. 6, pg. 83)
The narrow path had opened suddenly onto the edge of a great black lake. Perched atop a high mountain on the other side, its windows sparkling in the starry sky, was a vast castle with many turrets and towers.
'It' is used when we mention what we had just said in the previous sentence, so I thought 'it' here means a mountain because there's nothing else that can be 'it' in 'Perched atop a high mountain on the other side'.
But what is 'mountain's windows' then? Does a mountain have windows?