...I, for instance, am proud; that is to say, impregnable... 
Alexandre Dumas, Pere 「Ten Years Later」
... Impregnable city ofPal-ul-don—alone of all the cities it has never been entered by afoeman since it was built there while Jad-ben-Otho was a boy...
Edgar Rice Burroughs 「Tarzan the Terrible」
...Built up at haphazard, all these things make an impregnable wall...
J. Henri Fabre Alexander Teixeira de Mattos 「The Life of the Fly」
...To resist the rigours of the winter, this surface must be impregnable...
J. Henri Fabre 「Bramble-bees and Others」
... These pear-shaped hollows form splendid lodgings, impregnable strongholds, in which the Osmiae find a safe and commodious retreat for their families...
J. Henri Fabre 「Bramble-bees and Others」
...The city was built at the foot of a natural rock which rises precipitously from the plain, and must have formed an impregnable stronghold against the attack of the foe...
L.W. King and H.R. Hall 「History Of Egypt, Chaldæa, Syria, Babylonia, And Assyria In The Light Of Recent Discovery」
...There has thus been a gradual tendency todrift from readily accessible locations to the most impregnable sites,which indicates how severely the Hopi must have been harassed bytheir foes...
Jesse Walter Fewkes 「Archeological Expedition to Arizona in 1895」
...The builders evidently placed the greatestreliance on their impregnable site, and freely adopted such arrangementas convenience dictated...
Victor Mindeleff and Cosmos Mindeleff 「A Study of Pueblo Architecture: Tusayan and Cibola」
...They had no great name touphold; no riches to expend in impregnable walls and towers...
Elise Whitlock Rose 「Cathedrals and Cloisters of the South of France, Volume 1」
...The tower episode solidified my power, and made it impregnable...
Mark Twain (Samuel Clemens) 「A Connecticut Yankee in King Arthur’s Court, Complete」
...With such a defensethe Capitol and its grounds seemed impregnable...
Various 「Astounding Stories, June, 1931」
...An impregnable fortress of spines, the thing drifted slowly awaythrough the twisted loops of cable...
Various 「Astounding Stories of Super-Science September 1930」
...o the Ralas have ruled from this impregnable amphibian city...
Various 「Astounding Stories of Super-Science, August 1930」
..."You know that amphibian city is almost impregnable because humans canhardly live long enough under the water to get into it, let alonefight under water as the frog-men can...
Various 「Astounding Stories of Super-Science, August 1930」
...Saret Balisle seemedto be impregnable...
Arthur J. Burks 「The Mind Master」
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