Cover

Walls, trees, creatures, and other obstacles can provide cover during combat, making a target more difficult to harm. A target can benefit from cover only when an attack or other effect originates on the opposite side of the cover.
There are three degrees of cover. If a target is behind multiple sources of cover, only the most protective degree of cover applies; the degrees aren't added together. For example, if a target is behind a creature that gives half cover and a tree trunk that gives three-quarters cover, the target has three-quarters cover.

Half Cover

A target with half cover has a +2 bonus to AC and Dexterity saving throws. A target has half cover if an obstacle blocks at least half of its body. The obstacle might be a low wall, a large piece of furniture, a narrow tree trunk, or a creature, whether that creature is an enemy or a friend.

Three-Quarters Cover

A target with three-quarters cover has a +5 bonus to AC and Dexterity saving throws. A target has three-quarters cover if about three-quarters of it is covered by an obstacle. The obstacle might be a portcullis, an arrow slit, or a thick tree trunk.

Total Cover

A target with total cover can't be targeted directly by an attack or a spell, although some spells can reach such a target by including it in an area of effect. A target has total cover if it is completely concealed by an obstacle.

Cover in Mass Combat

In Mass Combat, units are considered covered if another unit stands between an attacker and its target. Any unit targeting another with an engage or volley order when a third unit is between them in the way is considered covered, and the engage and volley orders are broken.
Some [units](article:5e830465-3a08-4226-82f1-e177fc95ce8d) have "Volley 1," "Volley 2," or "Volley 3," special abilities that allow them to skip over multiple levels of cover.

Cover in Fortifications

Those atop or behind the walls may receive partial or full cover, at the discretion of the GM, and defenders may also receive Advantage when attacking enemies below.
In Mass Combat, city walls, fortresses, and buildings offer fortification bonuses to defending units:

• Units behind walls count as being covered by three units.
• Ranged units on behind walls, such as Light or Heavy Archers have 1 range level added to their volleys.