In fortification, cascans, or cascanes, are holes, or cavities in the form of wells, made in the terreplein, near a rampart; from which a gallery, dug in a similar manner underground, is conveyed, to give air to the enemy's mine.
This article incorporates text from a publication now in the public domain: Chambers, Ephraim, ed (1728). "article name needed". Cyclopædia, or an Universal Dictionary of Arts and Sciences (first ed.). James and John Knapton, et al.
The original article can be found at Cascan and the edit history here.