Verified answer

これが海への通路だ。 correct

これが海へ通路だ。 wrong! We don't say this.

・・・への path; way

・・・へ +[行く；通じる；つながる] path; way

これが海へ通じる道だ｡

This is the path leading to the sea.

ここが図書館への入り口です｡

This is the way toward the entrance of the library.

ここが図書館へ行く通路です｡

This is the path leading to the library.

ここが図書館の入り口です｡

This is the entrance door of the library.