He mentions the foxes, because they were very numerous in those parts, and because in the late desolations the foxes did walk in the mount and city of Zion, Lamentations 5:18, wherewith he seems to upbraid them. He shall even break down their stone wall: it is so low, that a fox can easily get to the top of it; and so weak, and done so hastily and carelessly, that the least thrust will tumble it down.
Related Commentaries of Nehemiah 4:3
Nehemiah 4:3
3 Now Tobiah the Ammonite was by him, and he said, Even that which they build, if a fox go up, he shall even break down their stone wall.