John Gill Commentary Nehemiah 4:3

"Now Tobiah the Ammonite was by him, and he said, Even that which they are building, if a fox go up, he shall break down their stone wall." — Nehemiah 4:3 (ASV)

Now Tobiah the Ammonite was by him
Who was one of his brethren he spoke before, (Nehemiah 4:2) ,

and he said ;
in the like contemptuous and scoffing manner;

even that which they build, if a fox go up, he shall break down their
stone wall ;
signifying not only that it was so low that a fox could easily get up to it, or leap over it; but that the materials were so bad, and the work so poorly done, that the weight of a fox would break it down; of which creatures many were thereabout, since Jerusalem was desolate, see (5:18) .