Translation:Our hotel was as big as a castle.
Why is it "tiel...kiel..." and not "tiel...tiel..." ?
What's the reason behind the switch?
Because they are two different things: first you have tiel + adjective to show what quality you are comparing, the second is kiel + noun to show the standard for comparison.
The question is rather, perhaps, why English uses the same word for those two different notions, and not - say - "so big like a mountain" rather than "as big as a mountain".