pregnable
Open to attack
Pregnable means vulnerable to attack or capture—the opposite of impregnable.
A fortress with weak defenses is pregnable.
The word exists almost entirely as a back-formation from impregnable, which is far more common: The castle's crumbling walls left it pregnable.
You'll encounter it mainly in military or strategic contexts, often with a slightly literary flavor.
Open to attack
- breachableCapable of being penetrated
- conquerableAble to be vanquished
- penetrableCapable of being pierced
- defeatableAble to be beaten
vanquishable · siege · assault · storm · overrun · breach · infiltrate · sapper
Lacking protective measures
- unfortifiedLacking military defenses
- unguardedWithout protection or caution
- unprotectedWithout safeguard or shield
- exposedOut in open
vulnerable · weak · helpless · unarmed · unshielded · unprepared · open · uncovered
assailable
Open to attack
- breachableCapable of being penetrated
- conquerableAble to be vanquished
- penetrableCapable of being pierced
defeatable · vanquishable · siege · assault · storm · overrun · breach · infiltrate · sapper · escalade
undefended
Lacking protective measures
- unfortifiedLacking military defenses
- unguardedWithout protection or caution
- unprotectedWithout safeguard or shield
exposed · vulnerable · weak · helpless · unarmed · unshielded · unprepared · open · uncovered · unpatrolled
fortress
Heavily defended military structure
- castleFortified medieval residence
- citadelFortified urban stronghold
- strongholdFortified place of defense
bunker · stockade · palisade · redoubt · blockhouse · keep · outpost · turret · bulwark · entrenchment
- vulnerabilityvulnerability, vulnerable, defenselessness, defenseless, helplessness
pregnableimpregnable
Impossible to capture
invulnerable · unconquerable · invincible · impenetrable · inviolable