缸口大的門 A Door The Size Of A Jar
咦！約有十餘里，就到了那大石邊。行者指那缸口大的門兒道：「兀的便是也。」天王道：「『不入虎穴，安得虎子。』咦！约有十余里，就到了那大石边。行者指那缸口大的门儿道：「兀的便是也。」天王道：「『不入虎穴，安得虎子。』
逐句解 Line by line
Ai!
The third of the chapter's interjections, and the shortest transition in the book: one character and the company has crossed the mountain.
It was some ten lǐ or more, and they came to that great rock.
約有十餘里 is about ten lǐ and more, so roughly six kilometres. The novel gives the distance because the whole expedition is about scale, and this is the last plain measurement in the chapter.
Pilgrim pointed at the door in it, the size of a jar mouth, and said, "That is the one."
缸口大 is the size of a jar mouth — a great earthenware water jar, so a hole you would have to stoop to. It is the entrance to a hundred and seventy kilometres of country, and the mismatch between the door and what lies behind it is the point of the description. 兀的 is a vernacular this here, a pointing word that has since dropped out of the standard language.
The Heavenly King said, "If you do not enter the tiger's den, how will you get the tiger's cub?
不入虎穴 and 安得虎子 make a real saying with a real source: it is what Ban Chao said to his men in the Later Han before a night attack, and it has been the standard thing to say before a rash entry ever since. English says nothing ventured nothing gained and loses both the tiger and the den. 得 is dé, to get, and 子 keeps the light zi of an animal's young.
The great heavenly cordon arrives at its objective and finds a hole the size of a water jar. The novel does not comment; it simply puts the two facts next to each other.
The Heavenly King reaches for a saying from a famous night raid, and it is the fourth or fifth time in this chapter that somebody has settled a question by quoting one. Nobody in this book argues if a proverb will do.