Complex sentence: The moat is in turn surrounded by a sloped grassy area or glacis, cleared of trees, that provided no cover for any advancing enemy forces and would have retarded or stopped cannon shot from naval ships.
Split into simpler sentences: The moat is in turn surrounded by a sloped grassy area or glacis, that was once cleared of trees, providing a clear field of fire at any advancing enemy forces. The slope would have also retarded or stopped cannon shot from naval ships.