Charles Ellicott Commentary 1 Chronicles 9:18

"who hitherto [waited] in the king`s gate eastward: they were the porters for the camp of the children of Levi." — 1 Chronicles 9:18 (ASV)

They were porters in the companies of the children of Levi. —Rather, They are warders for the camps of the sons of Levi. (Compare to Numbers 3:23 and following, where it is prescribed that the Levites encamp on the four sides of the tabernacle.) The primitive terminology is used in order to convey the idea that the Levitical wardership of the Temple went back historically to that of the Mosaic sanctuary.