Fort, fortress, citadel, stronghold, fastness denote in common a structure or place offering resistance to a hostile force.
A fort is an enclosed, fortified structure occupied by troops.
A fortress is a large fort of strong construction intended for long-term occupancy (as on the border of a hostile country).
A citadel is a fortification, usually on an eminence, that protects a city or keeps it in subjection.
A stronghold is a strongly fortified place whose resistance to attack or siege affords protection to its occupants.
A fastness is a place whose inaccessibility or remoteness makes for security. It may or may not be fortified.
These terms often have extended use. In such use a fort is something that by its very nature resists attack and a fortress is something that gives a feeling of security.
Citadel and stronghold are very similar in their extended uses, both being applied to a place where or, sometimes, to a class or group in which something prevails or persists in spite of attacks or encroachment.
Fastness characteristically suggests impenetrability or inaccesibility.