Or, a border, raised up above the rest of the ark, as a crown is above that which it is applied to, only a crown is round, and this was square. This was both for ornament, and for the fastening of the covering of the ark to it.
Related Commentaries of Exodus 25:11
Exodus 25:11
11 And thou shalt overlay it with pure gold, within and without shalt thou overlay it, and shalt make upon it a crown of gold round about.