Corner pieces in chess are essential for the game, and they include ROOKS (5 letters), ELLS (4 letters), and Castles (7 letters). ROOKS are the most powerful pieces on the board, and they can move any number of squares horizontally or vertically. ELLS are the second most powerful pieces, and they can move any number of squares diagonally. Castles are the least powerful pieces, and they can move one square horizontally or vertically. All three pieces are important for controlling the board and protecting the King.