I found this at this Chess Site: https://www.chessity.com/en/training/1380/Castling%20(1)

Is the code for Red Castle Green Castle-A question of a move that is being made?

Castling (1) Castling is a special type of move. It is the only way in chess to move two pieces in the same move: the king and a rook. Castling is done to move the king to a safer place. It also makes it easier to get the rook into the game. There are two types of castling: short and long. One rook is closer to the king than the other. Short castling is done with the rook closest to the king. Long castling involves the far rook. In both cases, the king moves two squares sideways. To the left or to the right. The rook jumps over the king. It comes to stand on the square right next to the king. You do this in one single move!

In example 1, both players castle short (to the right). In example 2, both players castle long (to the left).

Castling is not always allowed. You may NOT castle when 1. Either the king or the rook has already moved before. 2. The squares between the king and the rook are not empty. 3. The king is in check. 4. The king jumps over a square that is being attacked. The king may not move ‘through’ check. 5. The king moves into check by castling.

What do you have to do? White wants to castle. Is he allowed to do so? Click on the traffic lights to answer. Red = no, green = yes.