Complex sentence: Most of the characters are chess pieces participating in a game on a giant board in which each square is about one square mile in size; similar matches played with human pieces on enormous fields often occurred in the Middle Ages and Renaissance.
Split into simpler sentences: Most of the characters are chess pieces participating in a game on a giant chess board in which each square is about one square mile in size. Similar chess games played with human pieces on enormous fields often occurred in the Middle Ages and Renaissance.