In this game, which I played on Gameknot against a player rated at the time in the lower 1600s (and with an all-time-high ranking of 1691), my opponent, playing Black, castled into danger on the kingside.
In the illustration, the critical moment has arrived. Black’s kingside pawn structure has been disrupted by an exchange of minor pieces, leaving an undefended Black pawn on g6. Now would not be a good time for Black to castle on the kingside, but that is exactly what Black did!
If you are playing Black, and White opens with “e4” (also known as the King’s Pawn Opening, or P-K4 in the old notation), one solid response that I prefer is to reply with “e6” – i.e. move your own King’s Pawn one space forward.
White’s second move is typically “d4” – i.e. moving the Queen’s pawn forward two spaces next to the King’s pawn. Black then plays “d5”, advancing his Queen’s pawn two spaces forward. Those are the opening moves of the French Defense (or French Defence in British English):