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.