The most common and most likely answer to this crossword clue is the 6 letter word DONJON
I've seen this crossword clue in The Times
D
O
N
J
O
N
More clues leading to the result DONJON
Buddy explains!

The straight part
We know that "tower" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case tower = donjon.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 DONJON mean?
DONJON noun- The fortified tower of a motte or early castle; a keep.