Adam Clarke on Exodus 30:2

Four-square - That is, on the upper or under surface, as it showed four equal sides; but it was twice as high as it was broad, being twenty-one inches broad, and three feet six inches high. It was called, not only the altar of incense, but also the golden altar, Num 4:11. For the crown, horns, staves, etc., see on the altar of burnt-offering, Exo 27:1 (note), etc.