After you go through Snowpeak Ruins, Yeta looks into the mirror and is corrupted. She turns into Blizzeta: Twilt Ice Mass.
in hyrule castle
there isn't one...
My opinion is ganondorf's first phase puppet princess Zelda
The last level on Zelda twilight princess is hyrule castle where you face the final boss ganondorf.
beat the boss
deffet the monkey mini boss
It's on the map.
Argorok the Dragon
The mirror of twilight is in the desert. you gain access through the mirror of twilight after defeating the boss of the desert temple.
Find the big key.
by beating the final boss
in the main room