taru-adjective, adverb taking the 'to' particle
Literary taru-adjective describing something physically or metaphorically unyielding, steadfast, or rigid. Often used in formal or written contexts.
彼の決意は牢固として揺るがなかった。
His determination was firm and unshakable.
その要塞は牢固たる守りを誇っていた。
The fortress boasted a solid defense.
The exact derivation is uncertain; the kanji 牢 (prison, firm) and 固 (hard, firm) combine to suggest a meaning of 'firmly fixed'.