In the short story The Unbreakable Alibi, the Castle Hotel is a hotel in Torquay. Una Drake is seen at the hotel at the same time as she is seen having supper at the Savoy Hotel in London. This forms the "unbreakable alibi" refered to in the title. In the story, Montgomery Jones engages Tommy and Tuppence to solve this mystery. According to the text, the Castle Hotel was "a big one overlooking the sea", which fits the description of the Grand Hotel.
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