Usage patterns
〈物〉が堅固だfor something (a structure) to be solid/firm
This castle wall is extremely solid.
意志が堅固だto have a firm, unwavering will
She has a firm will and never gave up on the plan halfway through.
堅固な守りa solid defense
The team blocked the opponent's attack with a solid defense.
堅固な城a formidable, well-fortified castle
This mountain castle is known as a formidably fortified castle.