The altar that is referred to here is the altar of burnt offering, which stood in the outer court. This was the altar where God gave forgiveness of sins to the person who brought a sacrifice (Leviticus 17:11). Now he has rejected this avenue of forgiveness.
The LORD gave his altar over to the enemy, together with the whole inside of the “sanctuary.” Heathens stormed through the temple with the sound of victory, “as on the day of festival."
7 The Lord has scorned his altar, disowned his sanctuary; he has delivered into the hand of the enemy the walls of her palaces; they raised a clamor in the house of the LORD as on the day of festival.