אִם־חוֹמָ֣ה הִ֔יא נִבְנֶ֥ה עָלֶ֖יהָ טִ֣ירַת כָּ֑סֶף וְאִם־דֶּ֣לֶת הִ֔יא נָצ֥וּר עָלֶ֖יהָ ל֥וּחַ אָֽרֶז׃
If she be a wall, We will build upon her a turret of silver; And if she be a door, We will enclose her with boards of cedar.
Reshit Chokhmah
Also in the Midrash they explained the verse “every man with his sword on his thigh” (Song of Songs 8:9) relating to being cautious of the Brit, “because of dread in the night” that they are cautious of sin, which is prevalent at night-time.