Thou shalt make an altar. Rather, "the altar." God had already declared that he would have an altar made to him in the place where he should "record his name" (Exodus 20:24). And, even apart from this, an altar would be regarded as so essential an element in Divine worship, that no place of worship could be without one.
Of shittim wood. God had required (1. s. c.) that his altar should be "of earth," or else of unhewn stones (Exodus 20:25). The command now given was to make, not so much an altar, as an altar-case (see Exodus 27:8).
There can be no doubt that Jarchi is right in supposing that, whenever the tabernacle for a time became stationary, the hollow case of the altar was rifled up with earth, and that the victims were burnt upon this.
Four-square. Altars were commonly either square or round. An Assyrian triangular one was found by Mr. Layard at Nineveh; but even this had a round top. The square shape is the most usual, and was preserved, probably in all the Temple altars, certainly in those of Solomon (2 Chronicles 4:1) and Herod (Joseph.
Bell. Jud. 5.5, § 6).