Very firm, solid, or stable; not easily broken or changed. Used for physical things (structures) and abstract things (relationships, systems).
強固な基礎の上に建てられた家は地震に強い。
A house built on a solid foundation is resistant to earthquakes.
長年の信頼関係によって二社の提携は強固になった。
The partnership between the two companies has become solid through years of trust.
その城壁は石で強固に作られている。
The castle walls are built solidly of stone.