| impregnable fortress | 0.86 | Ali, who fought the Soviets from Tora Bora, described it as an impregnable fortress. | ||
| impregnable island | 0.26 | The numerically inferior Japanese surprised British military planners by attacking the supposedly impregnable island from the rear and fighting through thick jungle. | ||
| impregnable position | 0.26 | It put the home team, winner of the first test, in a virtually impregnable position. |