C. F. Keil and Franz Delitzsch on Nehemiah 6:16

Neh 6:16

The news that the wall was finished spread fear among the enemies, viz., among the nations in the neighbourhood of Jerusalem (comp. Neh 4:1; Neh 5:9); they were much cast down, and perceived “that this work was effected with the help of our God.” The expression בעניהם יפּלוּ occurs only here, and must be explained according to פּניו יפּלוּ, his countenance fell (Gen 4:5), and לב יפּל, the heart fails (i.e., the courage) (1Sa 17:32): they sank in their own eyes, i.e., they felt themselves cast down, discouraged.

To this Nehemiah adds the supplementary remark, that in those days even nobles of Judah were in alliance and active correspondence with Tobiah, because he had married into a respectable Jewish family.