edit: plus what about web, etc.
From the SRD:
Line of Effect: A line of effect is a straight, unblocked path that indicates what a spell can affect. A line of effect is canceled by a solid barrier. It's like line of sight for ranged weapons, except it's not blocked by fog, darkness, and other factors that limit normal sight.
The character must have a clear line of effect to any target that the character casts a spell on or to any space in which the character wishes to create an effect. The character must have a clear line of effect to the point of origin of any spell the character casts. For bursts, cones, cylinders, and emanating spells, the spell only affects areas, creatures, or objects to which it has line of effect from its origin (a burst's point, a cone's starting point, a cylinder's circle, or an emanating spell's point of origin).
An otherwise solid barrier with a hole of at least 1 square foot through it does not block a spell's line of effect.
Such an opening makes a 5-foot length of wall no longer considered a barrier for purposes of a spell's line of effect (though the rest of the wall farther from the hole can still block the spell).
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So again, if the arrowslit is within range, you can cast your Web through it with no problem. You don't even need to see where you're aiming - you only need line of effect, not line of sight.
The advantage of imbue arrow is not accuracy, it's range.
-Hyp.