Franci de Castellvi - Narcis Vinyoles 18. ... Qg6??, a blunder by black? [Site "Valencia"]
 [Date "1475"]
 [White "Franci de Castellvi"]
 [Black "Narcis Vinyoles"]
 [Result "1-0"]
 [startply "36"]
 [FEN ""]

 1. e4 d5 2. exd5 Qxd5 3. Nc3 Qd8 4. Bc4 Nf6 5. Nf3 Bg4 6. h3 Bxf3 7. Qxf3 e6 8. Qxb7 Nbd7 9. Nb5 Rc8 10. Nxa7 Nb6 11. Nxc8 Nxc8 12. d4 Nd6 13. Bb5+ Nxb5 14. Qxb5+ Nd7 15. d5 exd5 16. Be3 Bd6 17. Rd1 Qf6 18. Rxd5 Qg6?? 19. Bf4 Bxf4 20. Qxd7+ Kf8 21. Qd8# 1-0


A notable game in chess history is Scachs d'amor, recording a game of chess in the 15th century close to the current rules of the game. It does so in the form of a poem (original text, English translation).

Most of the game shows reasonable play, if not exactly modern. But when it comes to this move, it simply seems pointless. Consider for example the alternative 18. ... c3, which ultimately wins back some lost material for black, possibly giving better chances to secure a draw.

In any case, Castellvi quickly takes advantage of this, mating a handful of moves later.

What motivates 18 ... Qg6??

Some speculation:


Is it some (misguided) attempt at an offence by black, and in that case, what could Vinyoles have tried to achieve? Or is there some subtle difference from the modern rules, leaving this an acceptable move?
The game is somewhat artificial, acting as a tutorial for the rules of the game. Is this move necessary to show of an important rule to the reader?
Maybe it is for some kind of artistic purpose? The poem follows a strict form, and is divided in 64 stanzas. Does this awkward move somehow help achieving this form? Or does a formulation like "Queen’s check mate in the house of the other Queen" add something of value by allegory?
These players are just pretty weak. 5...Bg4 should already have been punished by Bxf7, many more blunders follow. 

Qg6 is a double attack against c2 and g2, that's probably motivation enough.