On the chiastic structure of Revelation
A chiastic outline of the book of Revelation was proposed by Nils Wilhelm Lund, in 1942.  Reviews of his book viewed Lund’s claims as too revolutionary.
Chiasm was used as a literary device in the ancient world, in Babylonia, Israel, Greece, and Rome. It fell out of use, however, and in modern times the existence of chiasms in ancient literature was only recognized by a few scholars in the 18th and 19th centuries. This changed in the middle of the twentieth century, when Nils Wilhelm Lund wrote Chiasmus in the New Testament. “Since these seminal studies the study of New Testament chiasm has blossomed, until today recognition of chiastic structure is common in full-scale commentaries and other scholarly works. The study of Old Testament chiasms has likewise begun to come of age.”
The above quote is from http://www.conservapedia.com/Chiasmus.
The outline below, showing the center or focus of the book of Revelation occurs in chapter 12, where Satan is cast out of heaven, and the saints are victorious, incorporates some of Lund’s ideas:
L.L. Thompson noted “circularity” or concentric development in the structure of Revelation, and wrote: 
The seer tends to develop his material concentrically into ever-widening rings. So, for example, several of the eschatological promises to those who conquer (the victorious)–an element of the seven letters of chapters 2-3–reappear in the vision of the New Jerusalem in chapters 21-22: at the beginning of the vision of renewal God makes the link to the letters by saying, “He who conquers shall have…” (21:7, cf. 2:11); the tree of life promised to those victorious at Ephesus (2:7) appears in the city (22:2); The victorious at Smyrna will not be harmed by the second death (2:11), a phrase that is made clear in 21:8 (cf. 20:6, 14); those conquering at Sardis will not have their names wiped from the book of life (3:5, cf. 21:27); and to those conquering at Laodicea Jesus promises a seat with him on his throne (3:21), while at 22:5 his servants reign forever in the city with the enthroned God and the royal Lamb.
Elisabeth S. Fiorenza proposed a chiastic-type correlation between Revelation 1:9-3:22 and 19:11-22:9. 
Warren Austin Gage supported Nils Lund’s chiastic correlation between the Seven Epistles of Revelation 2:1-3:22, and the visions of the whore of Babylon and the New Jerusalem, Revelation 17:1-22:5. In his thesis Gage developed and explored Lund’s idea. 
A. Introduction: apocalypse, epistle, prophecy (1:1-3, 4-8, 9-20) B. Vision: the saints on earth (chs. 2-3) C. The Heavenly Sanhedrin Convened for Judgment and Enthronement (chs. 4-5) D. Seven Seals (6:1-8:1) E. Seven Trumpets (8:2-9:21) F. Theophany: the Lord's messenger descending to sea and land (ch 10) G. War against the Saints on Earth (ch. 11) G' War against the Dragon in Heaven (ch.12) F' Counter-Theophany: Yamm's two messengers ascending from sea to land (ch. 13) E' Unnumbered Series of Seven Proclamations (ch. 14) D' Seven Bowls (chs. 15-16) C' The Heavenly Sanhedrin Convened for Judgment and the Messianic Reign (chs. 17-20; cf 20:4-15) B' Vision: the saints in heaven (21:1-22:5) A' Conclusion: apocalypse, epistle, prophecy (22:6-0, 10-20, 21)
Snyder’s scheme, IMO, misses evidence for chiasm in chapter 12.
David L. Barr stated,  “Many have observed a strong correlation between the beginning and ending of Revelation. There are at least eleven points of correspondence.” These are evidence for the outermost level of a chiastic structure. He listed these points:
John names himself [1:1, 4, 9; 22:8]
An angel sent [1:1; 22:6]
Will soon take place [1:1; 22:6]
The servants [1:1; 22:6]
Reader blessed [1:3; 22:7]
The time is near [1:3; 22:10]
Grace to you [1:4; 22:21]
The Alpha and Omega [1:8; 22:13]
The Spirit [1:10; 22:17]
Stars and angels [1:16, 20; 22:16]
John falls at feet [1:17; 22:8]
4. Warren Austin Gage, St John’s vision of the heavenly city. p. 26.