Left
阝 is the side-form of
阜 (mound, embankment) — the indexing radical. Originally
防 meant a dyke or earth wall built to hold back water; from physical defense came the abstract senses to guard against and to prevent. Same left-mound family as
阵 (battle line),
阶 (steps),
院 (courtyard). Note: right-side
阝 represents
邑 (city), a different morpheme.