fort

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

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here ... a famous robber had his stronghold— Ritchie

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A fastness is a place whose inaccessibility or remoteness makes for security. It may or may not be fortified

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a strong and almost inaccessible fastness— H. H. Wilson

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These terms often have extended use. In such use a fort is something that by its very nature resists attack

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oft breaking down the pales and forts of reason— Shak.

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and a fortress is something that gives a feeling of security

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my rock and fortress is the Lord— John Wesle

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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

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the very headquarters, the very citadel of smuggling, the Isle of Man— Burke

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{

the scientific world has been the very citadel of stupidity and cruelty— Shaw

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the South of Somersetshire, one of the strongholds ... of the Anglo-Saxon dialect— Jennings

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Fastness characteristically suggests impenetrability or inaccesibility

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in the impregnable fastness of his great rich nature he [the Roman] defies us— J. R. Lowell

}