Preposition: of

• earth: The Parliamentarians built breastworks of earth and stone to protect their musketeers and artillery pieces.

Converse of object

• form: On Gell's left flank, hedges and walls formed a natural breastworks for his musketeers.

Adjective modifier

• natural: On Gell's left flank, hedges and walls formed a natural breastworks for his musketeers.

Noun used with modifier

• timber: A low timber breastwork prevents landward migration of spit onto marshes.