King safety
A king stuck in the centre with open files nearby is a permanent tactical liability — even a pawn down, keep it stuck.
White to play
O-O
One move takes the king off the open e-file and brings the last undeveloped piece towards the centre.
What to look for
Ask whether the king has castled, whether the files beside it are open, and whether the pawns in front of it have moved. Two of those three going wrong is usually enough for an attack.
When it applies
In the first ten moves, castle unless there is a concrete reason not to. Later, before every pawn push near your own king — those pawns are the roof.
How it goes wrong
Castling into the attack is worse than staying put. If they already have pieces aimed at one side, castle to the other or leave the king in the middle behind a solid pawn wall.
Where it came up
Not cited in your current game.
Practise it
No positions yet. When you miss this idea in a game, the position it cost you shows up here — and in the practise set — with the answer hidden.