Answer: fortress

AnswerCrossword Clue
FORTRESSThe Tower of London, for one
FORTRESSStronghold
FORTRESSThe Tower of London, once
FORTRESSMilitary stronghold
FORTRESSDefensive structure
FORTRESSMalta or Gibraltar.
FORTRESSFlying ....
FORTRESSStronghold.
FORTRESSCitadel
FORTRESSSiege site
FORTRESSCastle
FORTRESSKeep
fortressA person or thing not susceptible to outside influence or disturbance
fortressA heavily protected and impenetrable building
fortressto fortify (to strengthen against attack)
fortressA military stronghold, esp. a strongly fortified town fit for a large garrison