Why did my opponent resign here instead of castling? [spell chess analysis]

https://www.chess.com/variants/spell-chess/game/54016480/9/3

Since there is no engine, I'm trying to analyze my game. Here, I don't understand why my opponent resigned. If I were them in the final position, I would simply castle, and after a lot of trades we end up with a 4 vs 5 piece endgame which I should have an advantage. Or at least, that is my guess. Where did this game go wrong? Why is black allowed to achieve this in spell chess? Does my opponent need to play a more forcing line or freeze first?

Thanks in advanced for the analysis. Note this is spell chess.

Even g3 instead of nf3 has a similar result too, no? Is there a move that saves white in this line? Or is black simply forcing white's timeliness with their attack right from the get-go in spell chess? Lots of things to consider here.

Any thoughts? I guess people aren't so interested in spell chess. I think it's a good variant, though it should be played to mate instead of capture in my humble opinion.

To

Hi

?

Who cares

you cant castle through check...

This is mine: https://www.youtube.com/watch?v=4cTSXmxci_0

no you cant castle through check