- impenetrable by cannonballs
- 1948, Frances Winwar, The Saint and the Devil, page 33:
- It had not helped the duke to build himself a cannonproof stone chamber to sleep in for dread of vengeance after the assassination of Orleans.
- 1983, Richard J. Hargrove, General John Burgoyne, page 26:
- A cannonproof breastwork, built during the previous war, extended along the beach from the hills to the rocks.