8 
7 
2 
a 
b 
c 
d 
e f g 1 h Why it works
Black has done the hard part: accepted both Danish pawns, traded a pair of knights and even swapped the dark-squared bishops into White's b2-bishop. It feels like time to tidy up, and 10...O-O?? tidies the king straight into the crossfire. 11.Qg4! creates an unanswerable double grip on g7: queen from g4 and bishop from c3 both hit the one square only the king defends. The engine score after castling is not 'worse'; it is a collapse (+17 within a few moves in the line). Black was already under pressure; castling was the one move that lost instantly.
Refutation
The only fight was 10...Qg5!, offering the queen trade that kills the g7-battery before it exists. White is still clearly better after 11.Qxg5 (the two open files and the monster on c3 are worth more than Black's extra pawns), but there is no forced disaster. In the Danish accepted, Black's king often has no safe address at all; that is the gambit's real price.