Revelation is a book of symbols. Whenever we read Revelation we should be asking ourselves, “what do these symbols mean?” We should be cautious of assuming that some has the same meaning in Revelation as it might elsewhere.
Elsewhere in the Bible, “heaven” often refers to the places where God dwells. In Revelation, it might not mean the same thing. For example, the opening verse: “a great sign appeared in heaven: a woman clothed with the sun” (Rev 12:1). Now there isn’t really a woman clothed with the sun (that’s a symbol), and so the “heaven” in which this sign appeared was probably not literal heaven, but symbolic heaven.
So what might “heaven” mean in this context? Some interpreters think that “heaven” refers to the political heights. To be “cast down from heaven” would mean to lose your position of political power.
However the interpretation I prefer is that “heaven” refers to those who belong to God (cf. “our brothers” Rev 12:10). They were previously “accused day and night before our God” by the Dragon (i.e. by sin), but that “accuser” has now been “cast down”. That is, believers are no longer accused in the presence of God by their sinful nature because that has now been atoned for. “They have conquered him by the blood of the Lamb” (Rev 12:11)