I do wonder what game state could've possibly been strongly in her favor and wholly removed her chances with one castling, not saying it's never happened but that seems a bit... immediate.
Watch the anime! They actually managed to construct a position in which:
- Castling wins (because the castled rook checks Monica's exposed king, initiating a winning tactical combination)
- Just moving the rook without castling loses (because the combination relies on a knight currently pinned to Elliott's king, if memory serves)
- Everything else also loses
The anime adaptation receives nothing less than full marks for how they handled the chess in Silent Witch. The manga, sadly, avoids showing Monica vs. Elliott almost entirely. Serializations progress on strict timetables, so it's likely they couldn't afford the legwork. (On page 19, a board was set up wrong. In chapter 6, two black pieces didn't get colored in and ended up white. I didn't miss it, because I'm just as tedious about chess as Lana is about fashion and Monica is about numbers, but an assistant who does their research diligently and is working from good references, but doesn't play chess, certainly might.)
The specific set of circumstances described in the light novel (winning by castling, losing by everything else) is so unlikely it probably hasn't ever happened, but to clarify: in general terms, a game state that has been completely in favor of one player for dozens of moves flipping entirely, out of nowhere, because of one specific move, not just happens all the time, the fact it's possible is half the reason people play chess at all. A move like that would be called a brilliancy, or a blunder, or in Elliott's case, a dick move. Chess players dream of playing brilliances, and they live in eternal terror of committing blunders. It's all part of the greater whole. Except for the dick moves, we can do without those. For fuck's sake, Elliott...