Kulmbach was also burned as a Protestant town by imperial troops, but the Plassenburg proved impregnable.

However, by then, the German defences at the 2nd line were strengthened and proved impregnable.

At their inception in 1915, the British Mark I tank proved nearly impregnable to standard rifle fire.

It proved impregnable against both our photon torpedoes and our phasers.

The fortified cities proved impregnable to the Hanthawaddy forces.

This was restored and strengthened in 1740 into a fortress that proved impregnable up until the 20th century.

Once the great stone fortresses dotting their route of escape were garrisoned, argued Pizarro, they would prove impregnable.

Five spades was doubled and proved impregnable.

Otherwise its location atop a high bluff overlooking the Mississippi River proved impregnable to assault by federal troops.

One began to suspect that his technique would prove impregnable.