stronghold

n. fortified place; safe refuge

A stronghold is a strongly fortified defensive structure. The history of fortified buildings extends from antiquity to modern times. In medieval Europe, the castle was the most common type of stronghold and often the residence of the king or territorial lord. The Maginot Line is an example of a 20th century stronghold. See also fortification.
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Noun
1. a strongly fortified defensive structure
(synonym) fastness
(hypernym) defensive structure, defense, defence
(hyponym) bastion, citadel

(n.)
A fastness; a fort or fortress; fortfield place; a place of security.
  

Synonyms and related words:
acropolis, asylum, bastion, beachhead, bird sanctuary, blockhouse, bridgehead, bulwark, bunker, castle, citadel, donjon, fasthold, fastness, forest preserve, fort, fortification, fortress, game preserve, game sanctuary, garrison, garrison house, harbor, harbor of refuge, harborage, haven, hold, keep, martello, martello tower, mote, motte, peel, peel tower, pillbox, port, post, preserve, rath, redoubt, refuge, safe haven, safehold, sanctuary, snug harbor, strong point, tower, tower of strength, ward