The door to heaven is standing open for John (4:1). Opened by the Key of David? "...Come up here and I will show you what must take place after this." (4:1).
Regardless how bad things become, God is still on the throne. Surrounded in this case with 24 other thrones -- perhaps symbolizing all of God's People: the union of the 12 Tribes of Israel and the 12 Apostles of Jesus.
Through out the book of Revelation the scene keeps on shifting between glimpses of what is going on in heaven and what is happening on earth.
Sea of glass in 15:2 also.
Isaiah chapter 2 an example?
There seems to be a lot of singing going on in Heaven. Psalms: Ballad a song or poem that tells a story in short stanzas and simple words.