The answer
The Coptic altar takes the shape of approximately a cube, which resembles Christ’s tomb.
It is located right in the middle of the sanctuary, to resemble the heavenly altar: “I heard a voice from the four horns of the golden altar” (Rev. 9:13).
It is often made of stone, marble, or brick, but exceptions do exist.
It must be hollow so that saints’ relics can be kept inside or beneath it.
On the eastern side of the altar, there exists a small opening showing an interior recess or cavity, which was used during persecution to hide the Holy Gifts when necessary.
The altar is never raised above the sanctuary but is fitted directly on its ground, as directed by the divine commandment (Ex. 20:26).
It does not have any form of engraving, not even crosses (Ex. 20:25).
Multiple altars in the same church were introduced to fulfill the needs of the ever-growing Christian congregation, since it is forbidden to pray more than one liturgy on the same altar in the course of a single day.
Fr. Tadros Yacoub Malaty