堅固 is read as けんご (kengo) and means solid.
けんご
kengo
けんろうけんご
firm and solid
けんさいこるい
impregnable fortress
ようじんけんご
very cautious (watchful, vigilant)
どうしんけんご
being of firm (unshakable) faith and strict practice
ようがいけんご
(a fortress being) impregnable
いしけんご
strong determination
どうとくけんご
of strict morals (moral principles)
こんごうけんご
しそうけんご
being faithful to one's principles
ていそうけんご
chaste (usu. to one's husband)
かたい
obstinate
けんじつ
steady