The most common and most likely answer to this crossword clue is the 8 letter word HACIENDA
H
A
C
I
E
N
D
A
More clues leading to the result HACIENDA
- Heartless Havana spies bringing death inside ranch house
- Eccentric had a nice ranch — in Mexico, possibly
- Had a nice trip
- Harry had a nice big estate in Mexico
- I can head out for estate
- Criminal had a nice estate in Spain
- French company gets assistance around a farmstead
- Article supporting new echidna farm
- This house could require a Mexican wall!
- Alien had a nice estate
Buddy explains!

The straight part
We know that "castle" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case castle = hacienda.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does HACIENDA mean?
HACIENDA noun- A large homestead in a ranch or estate usually in places where Colonial Spanish culture has had architectural influence.