The most common and most likely answer to this crossword clue is the 9 letter word CASEMATES
C
A
S
E
M
A
T
E
S
Buddy explains!

The straight part
We know that "strengthened chambers" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case strengthened chambers = casemates.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 CASEMATES mean?
CASEMATES noun- A bombproof chamber, usually of masonry, in which cannon may be placed, to be fired through embrasures; or one capable of being used as a magazine, or for quartering troops.
- A hollow molding, chiefly in cornices.