The top was made hollow like a grate, that the ashes might fall through it. The crown was a border which encompassed the altar, that the things laid on it might not fall off.
Related Commentaries of Exodus 30:3
Exodus 30:3
3 And thou shalt overlay it with pure gold, the topa thereof, and the sides thereof round about, and the horns thereof; and thou shalt make unto it a crown of gold round about.