The Tower is the fourth major expansion for the Carcassonne game. Tower segments are added to the game, as well as some new actions. Instead of placing a Meeple as usual on their turns, players can now either place a tower segment on a tile with a space for a tower, add a tower segment to a tower already standing, or place a Meeple on top of a tower (which "finishes" it).
Placing a tower segment allows you to capture other players' Meeples, so long as they are in view of the tower in an orthogonal direction. If the tower is 1 piece high, they can capture Meeples that are within 1 tile of the tower; if the tower is 2 pieces high, they can capture Meeples within 2 tiles of the tower; etc. Not only do players lose whatever they may have scored with that Meeple, but they also have to pay 3 victory points if they want to free their Meeple.
Here are the full rules for 'Black Tower'. When a player constructs a black tower, the towers capturing properties switch and have a diagonal capturing ability. They can be reversed, and towers can be permanently abandoned by 'capping' with a white tower piece. However, some time and investment are required to make the new tower pieces.