【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.