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The word "fortress" includes: bunker, fortress, castle, stubborn fortress, bunker, fortress wall, etc.

Bunker refers to: multi-layer fortifications protruding from the ground, used for observation, shooting and garrison. Most of them are masonry or concrete structures.

The meaning of the fortress:

1, a sturdy building for defense in key locations.

2, metaphor is difficult to break things or not easy to accept new things and new ideas.

Stubborn fortress is the basic explanation: metaphor for stubborn people.

Basic explanation of bunker: low fortifications for rifle and machine gun shooting. Generally, it is a masonry or reinforced concrete structure. Used to cover bridges, ferries or block streets, roads and open spaces.