Herein, what does a rook symbolize?
In chess the castle is a major piece, now usually called a rook. Originally, the rook symbolized a chariot. The word rook comes from the Persian word rukh meaning chariot. Persian War Chariots were heavily armoured, carrying a driver and at least one ranged-weapon bearer, such as an archer.
Subsequently, question is, why is the rook important? Rooks play an important role in supporting the movement of your other pieces. Rooks are very effective in supporting the advance of your pawns. In the endgame rooks are effective in cutting-off the opponent's king from the action.
Keeping this in view, why does the rook look like a castle?
The piece we call a rook was considered to be a chariot rather than a castle, probably because of the speed with which it moves. The Sanskrit word for chariot was "ratha". In Middle English the concept of tower eventually turned into castle since most castles had towers.
Can a rook become a queen?
Under US Chess Federation rules and in casual play, an upside-down rook may be used to designate a queen (Just & Burg 2003:16–17).
