And I commanded the Levites … that they should come and keep the gates. Assigning the duty to his servants was probably a temporary arrangement. The permanent charge was committed to the Levites, who had been intrusted with the duty when the gates were first set up (Nehemiah 7:1).
They were to "cleanse," or purify, themselves, because the charge was considered a sacred one. Remember me, O my God, concerning this also. Compare Nehemiah 13:14. And spare me. It is worthy of notice that Nehemiah does not regard his good deeds as sufficient for his justification, but throws himself unreservedly on God's mercy.