An altar — Not for a monument of the victory, but for sacrifices, as the next words shew. There might be in that place more altars than one, when the multitude of sacrifices be required, which was the case, 1 Kings 8:64, and probably at this time, when all the tribes being met, they had many sacrifices to offer, some in common for all, and some peculiar to every tribe.
Related Commentaries of Judges 21:4
Judges 21:4
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.