"Our hotel was as big as a castle."
Translation:Nia hotelo estis tiel granda kiel kastelo.
The hotel is big in THAT way, in WHICH a castle is.
"That" suggests a Ti-correlative, and "which" suggests a Ki-correlative.
So, the short answer, cuz that's how that construction goes.
The longer answer:
The first is 'tiel' because we're providing an answer to a 'kiel' question ("how big was the hotel? The hotel was so big...")
The second is 'kiel' because we're starting a dependent clause, and those always begin with 'k..' words. In this case the clause is trivial, and we don't needlessly repeat in Esperanto any more than we would in English ("Our hotel was as big as a castle is big")