Jamieson, Fausset and Brown on 1 Chronicles 9:18

18. the king's gate--The king had a gate from his palace into the temple (2Ki 16:18), which doubtless was kept constantly closed except for the monarch's use; and although there was no king in Israel on the return from the captivity, yet the old ceremonial was kept up, probably in the hope that the scepter would, ere long, be restored to the house of David. It is an honor by which Eastern kings are distinguished, to have a gate exclusively devoted to their own special use, and which is kept constantly closed, except when he goes out or returns (Eze 44:2). There being no king then in Israel, this gate would be always shut.