Albert Barnes Commentary Judges 21:4

"And it came to pass on the morrow, that the people rose early, and built there an altar, and offered burnt-offerings and peace-offerings." — Judges 21:4 (ASV)

It is not certain whether the brass altar was at Bethel at this time, or whether it may not have been elsewhere, for example, at Shiloh with the tabernacle. Some, however, think that the altar here mentioned was “additional” to the brass altar, in consequence of the unusual number of sacrifices caused by the presence of the whole congregation (Compare to 1 Kings 8:64, see note).