These arrow slit items won't always be described as "an arrow slit". There might be the top of a tower with one in it described as "the tower edge", that you can FIRE or CAST at in order to hit baddies that are outside the tower, for instance.
Some types of arrow slit items may prevent certain types of ranged attacks. It would be impractical to sling stones through a literal arrow slit, for example.
Examples!
As can be seen in the example, you don't see the exact amount of damage or any additional effects, just that the target was hit. This is partly because it'd be finicky code-wise to make all of that viewable with this mechanic, but also, I think it kind of makes sense when you're firing at distant targets through an arrow slit that you don't see all the exact details of the results.>look arrow slit
You take a closer look at an arrow slit in the area ...
You see nothing particularly unusual about the arrow slit.
Looking through the arrow slit you see ...
[Fortress Approach] (OutMt) Noon, Mild, Clear, Calm
Small bushes and ferns interspersed with the occasional cedar tree line the mountain path leading toward a tall stone fortress.
The area is mostly quiet.
Also here: an infested shock-trooper (x35).
Obvious paths: none.
fire slit
You loose an arrow through an arrow slit at an infested shock-trooper with your longbow! Ranged(d100):37 vs Dodge(d103(2x)):20 = 17 (17%)
A hit to the neck!
(energy -10)
[RT was: 7]
sorc
Inky black tendrils snake and writhe around your hands as you summon nether to your command.
(Type release to stop channeling.)
[RT was: 3]
cast slit
You hurl a small inky-black glob through an arrow slit at an infested shock-trooper.
A small inky-black glob surges toward an infested shock-trooper. Ranged(d400):225 vs Dodge(d103(2x)):77 = 148 (37%)
A hit to the chest!
[RT was: 6]
Now to prepare some invasions in order to test this out.