Adam Clarke on Zechariah 9:11

As for thee also (Jerusalem) by the blood of thy covenant - The covenant made with Abraham, Isaac, Jacob, and the Israelites in general, and ratified by the blood of many victims; until the time should come in which the Messiah should shed his blood, as typified by the ancient sacrifices.

I have sent forth thy prisoners - Those who were under the arrest of God's judgments; the human race, fast bound in sin and misery, and who by the pitifulness of his tender mercy were loosed, he dying in their stead.