MB4 198b: Karmelis on Shabbos (345:14-15)

Definition of a karmelis: an area accessible to the public but is not generally used by the public (e.g. a lake, an ocean, or a בקעה/area of fields)

An roofed area that would otherwise be a reshut harabim if the roof is at least 4x4 tefachim is a karmelit.

A mound that isn't 4x4 or 10 tefachim high.