Wall¶
When an ability creates a wall, that area is expressed as "X wall." The number X is how many squares are used to make the wall. When you place a wall, you can build it one square at a time, but each square must share at least one side (not just a corner) with another square of the wall. A creature or object must be within the area to be targeted by a wall ability.
You can stack squares on top of each other to make the wall higher. Unless otherwise stated, a wall can't be placed in occupied squares, and a wall blocks line of effect.
Straight Lines
Talking about a straight line area of effect on an encounter map doesn't mean the line can only be a straight vertical or horizontal line of contiguous squares. It means each square in the line's length must move in the same direction without bending back in an opposite direction. To make a line area quickly, pick your line's origin square, then pick each subsquent square in the line one at a time in a single direction without bending back in an opposite direction.
Likewise, abilities and effects that require a creature to move in a straight line, such as the Charge main action or forced movement that is a push or a pull, don't have to take the form of a straight series of squares on the grid. Simply move the creature one square at a time in a single direction without ever bending back in a direction opposite to where they've already moved.