The most common and most likely answer to this crossword clue is the 10 letter word TERREPLEIN
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More clues leading to the result TERREPLEIN
Buddy explains!

The straight part
We know that "top of rampart" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case top of rampart = terreplein.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 TERREPLEIN mean?
TERREPLEIN noun- The sloping earthen embankment behind a defensive wall.
- The level platform atop a wall, typically protected by a parapet and (strictly) distinguished from the slightly higher banquette used by its defenders.
- Any level base used by artillery in the field.
- Any earthen embankment with a broad level top, such as those used for a canal elevated above a valley floor.