My partner's dad was evil: he taught "simple" versions of games, leaving out complicated rules like castling, when she & her sister were small, and then never explained the full rules later.

Thus, when she played me at uni, she was furious that I was "cheating," until I showed her the full rules (the internet is brilliant) and she realised that it was her dad being crazy.

In summary; it is essential to know castling exists, because otherwise as soon as you play another human, they will beat you with it.