Source : Webster's Revised Unabridged Dictionary (1913)

Expugnable \Ex*pug"nable\ ([e^]ks*p[u^]g"n[.a]*b'l), a. [L.
   expugnabilis.]
   Capable of being expugned.

Source : WordNet®

expugnable
     adj : susceptible to being stormed [syn: {stormable}]