So, I was just adopting my guy @ShadowCellen(not my alt, and sry). And in one of the game he just freak out and just give up lol, so I ended up checkmating him with long castle, and got this achievement.
The legit first ever time I have done this achievement. There is another achievement which is to checkmate with a king move(discover check with king and mate).
I believe over 80% of chess.com don't have this achievement xD
How to get the victory castle achievement:
1. Play with your closest, weakest friend (before that, ask your friend to come to your house or you come to your friend's house) in chess.com of course
2. Play seriously against your friend (if you found checkmate in the opening or midgame, don't checkmate your friend right away, wait till you got at least K, 3R v K)
3.
I have delivered checkmate with discovery attack from my king 😎
Wow congrats i did that too