As the game starts, a green rook stands exactly on e4.
The rook acts as a block. Nobody except a knight can jump over it - it cuts the lines of pieces. It can never be captured. It captures nothing and does not give check.
Either player, in his move, can choose to move the green rook. It moves like any rook.
A player may not move the green rook in such a way as will undo his opponent previous move! eg if I play green rook e4 to h4, you cannot answer me on your next move, green rook h4 to e4. You can move it anywhere else, or wait for another move, and then put it on e4 if the move is still legal.
No pawn can promote to a second green rook, though. One such barrier is enough. A very old idea of mine, which is nice to tell over these pages. Haven't told many people about it yet..
I should try that, cool idea.
I always liked the idea of beginning the game with a curtain dividing the board between your half and the opponent's half. You can setup the pieces however you want on your half of the board and your opponent can do likewise -- neither person being able to see the other person's setup. Then you raise the curtain and play the game. I never really figured out what would be the best arrangement of pieces.