Albert Barnes Commentary Nehemiah 7:1

"Now it came to pass, when the wall was built, and I had set up the doors, and the porters and the singers and the Levites were appointed," — Nehemiah 7:1 (ASV)

Since the watch of the temple had until now been kept by porters, singers, and Levites (1 Chronicles 26:1–19), so now the watch of the entire city was committed to men of the same three classes, their experience pointing them out as the most suitable persons.