Just my thoughts-

Castles- think Chess

Perhaps the castles are McMasters and Bolton.

RED_Castle= McMasters who has now left the team. LEFT could relate as in the port side of a ship has the red light. the traffic light on a city street is red when you have to stop.

Green_Castle= Bolton who now has the green light to go-clear sailing. right side of the ship = green light. Perhaps both of them relate to the game of Chess as Castles- which can move both vertically and horizontally on the board.

Q said to learn Chess…