Sforno on Numbers 18:3/ספורנו על במדבר י״ח:ג׳

1ושמרו משמרתך ומשמרת האהל, the Levites are to guard the Holy of Holies (structure) which in itself is your domain; they are also to guard all the other sacred locations outside the Holy of Holies.

2אך אל כלי הקודש, the ones housed within the Tabernacle, comprising the Table, the Lampstand and the golden altar,

3they must not approach, whereas the copper altar although it is outside the walls of the Tabernacle they must not approach either.