Charles Ellicott Commentary Exodus 27:1

"And thou shalt make the altar of acacia wood, five cubits long, and five cubits broad; the altar shall be foursquare: and the height thereof shall be three cubits." — Exodus 27:1 (ASV)

Thou shalt make an altar. —Hebrew, the altar. It is assumed that a sanctuary must have an altar, as worship without sacrifice was unknown. (Exodus 8:25–28; Exodus 12:27; Exodus 18:12; Exodus 20:24–26, etc.)

Of shittim wood. —This direction seems at first sight to conflict with those given in Exodus 20:24-25, where altars were required to be either of earth or of unhewn stone. But the explanation of the Jewish commentators is probably correct, that what was directed here to be made was an “altar-case” rather than an altar, and that the true altar was the earth with which, at each halt in the wilderness, the “case” of shittim wood covered with bronze was filled. (So Jarchi, Kalisch, and others.)

Foursquare. —Ancient altars were either rectangular or circular, as the square and the circle were regarded as perfect figures. A triangular altar was discovered by Mr. Layard in Mesopotamia, but even this had a circular top. In Hebrew architecture and furniture, curved lines were for the most part avoided, probably because they presented greater difficulties than straight ones.

The height thereof ... three cubits. —A greater height would have made it difficult to arrange the victims on the altar. Otherwise, the notion of perfection in form would probably have led to the altar being a cube.