Yahweh has done that which he purposed; he has fulfilled his word that he commanded in the days of old; He has thrown down, and has not pitied: He has caused the enemy to rejoice over you; he has exalted the horn of your adversaries.
That is, the enemies were in there cheering and yelling and all as they were destroying it, much as the voices and cheers and all that once went up in the days of their solemn feasts.