(A), in fortification, is a work having two faces and two flanks, all the angles of which are salient, that is, pointing outwards towards the country. The line of rampart which joins together the flanks of two bastions is technically called a curtain.
Bastions in fortifications were invented in 1480 by Achmet Pasha; but San Michaeli of Verona, in 1527, is said by Maffei and Vasari to have been the real inventor.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
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