I'm running a module that features a fort with arrow slits and murder holes, and I'm trying to figure out what the cover rules are for people on each side of them.
Under the Combat rules, the CRB states:
In some cases, such as attacking a target hiding behind an arrowslit, cover may provide a greater bonus to AC and Reflex saves. In such situations, the normal cover bonuses to AC and Reflex saves can be doubled (to +8 and +4, respectively). A creature with this improved cover effectively gains improved evasion against any attack to which the Reflex save bonus applies. Furthermore, improved cover provides a +10 bonus on Stealth checks.
From this, it seems clear that the defending archers should have improved cover. This is reinforced by this section under Dungeon Environments:
Walls with Arrow Slits
Walls with arrow slits can be made of any durable material but are most commonly masonry, hewn stone, or wood. Such a wall allows defenders to fire arrows or crossbow bolts at intruders from behind the safety of the wall. Archers behind arrow slits have improved cover that gives them a +8 bonus to Armor Class, a +4 bonus on Reflex saves, and the benefits of the improved evasion class feature. (emphasis added)
The words "defenders" and "behind arrow slits" make me think that the cover is at least somewhat directional - the defending archers are meant to be at an advantage over the besiegers (which makes sense). However, the general cover rules seem to suggest that the besiegers might also have some cover:
To determine whether your target has cover from your ranged attack, choose a corner of your square. If any line from this corner to any corner of the target’s square passes through a square or border that blocks line of effect or provides cover, or through a square occupied by a creature, the target has cover (+4 to AC).
When making a melee attack against an adjacent target, your target has cover if any line from any corner of your square to the target’s square goes through a wall (including a low wall). When making a melee attack against a target that isn’t adjacent to you (such as with a reach weapon), use the rules for determining cover from ranged attacks.
I think that the intent is that arrow slits would be at the corners of grid squares. RAW, I think that would give the besiegers no cover. However, on the map I'm using, the arrow slits are in the middle of the grid squares. RAW, I think that gives the besiegers at least cover, and maybe improved cover.
Distinct from this question (although related) in that this is about improved cover like arrow slits, whereas that was more a case of low cover.