From The Collaborative International Dictionary of English v.0.48 :
Assailable \As*sail"a*ble\, a.
Capable of being assailed.
[1913 Webster]
From WordNet (r) 3.0 (2006) :
assailable
adj 1: not defended or capable of being defended; "an open
city"; "open to attack" [syn: assailable,
undefendable, undefended, open]