Wall

A wall is any vertical surface. It is represented as follows:
 * Two vertices are defined for the endpoints.
 * A linedef specifies the "from" and "to" vertices, and one or two sidedefs.
 * Each sidedef names the wall texture(s) displayed on one side of the wall and the sector that that side faces.
 * The sector defines the floor and ceiling height (bottom and top of the wall).

A linedef, and thus a wall, can be one-sided or two-sided. One-sided walls display a single texture, and have no play area directly behind them. Two-sided walls can display a lower, middle, and upper texture (any of which may be transparent) to help create steps, windows and the like. The "middle" is determined by the floor and ceiling of the sector on the other side of the linedef.