Every morning and every evening the high priest burned fragrant incense on this altar as a symbol of Israel’s prayers (Exodus 30:7). Unlike with the altar of burnt offering (e.g., Leviticus 4:7; Leviticus 5:9), we read nowhere in the Old Testament of blood having to be poured out at the base of the altar of incense. Perhaps John conflates the two altars so that rituals of either altar are merged here. More likely, as the prayers of these martyred saints continue to go up (as incense rises), the source of these prayers are pictured as below the altar. Even in heaven these saints continue to plead with God.
9 When he opened the fifth seal, I saw under the altar the souls of those who had been slain for the word of God and for the witness they had borne.