[RPG] Did Spirit Guardians just wipe out the Orc arthe

As of the 2018 errata, if an enemy takes damage from your spirit guardians spell, your sanctuary spell ends

The description of the sanctuary spell (PHB, p. 272) originally said:

If the warded creature makes an attack or casts a spell that affects an enemy creature, this spell ends.

However, the 2018 PHB errata added "deals damage to another creature" to the list of conditions that end the sanctuary spell. The description of the sanctuary spell now reads (emphasis mine):

If the warded creature makes an attack, casts a spell that affects an enemy, or deals damage to another creature, this spell ends.

Even though the spirit guardians spell doesn't directly affect an enemy creature when cast, it does deal damage to them:

An affected creature's speed is halved in the area, and when the creature enters the area for the first time on a turn or starts its turn there, it must make a Wisdom saving throw. On a failed save, the creature takes 3d8 radiant damage (if you are good or neutral) or 3d8 necrotic damage (if you are evil). On a successful save, the creature takes half as much damage.

As a result, the sanctuary spell ends as soon as an enemy takes damage from a spirit guardians spell cast by the warded creature.

Even before the 2018 errata, the intent was that damaging an enemy would end the spell (though this was not reflected in the description of the spell). In an unofficial tweet in July 2016, rules designer Jeremy Crawford stated:

You first cast spirit guardians and then sanctuary. Does sanctuary end when spirit guardians deals damage to enemies?

RAW: Sanctuary isn't stopped if an already-cast spell deals damage. RAI: A damaging activity ends the spell.

RAI means errata or a future edition would possibly change RAW, right? Thanks for the answer

That's correct

Clearly, the 2018 PHB errata changed the spell so that it now works as intended.

Unless an AoE spell has a specific exception, it doesn't affect creatures through walls. To establish the type of spell Spirit Guardians is, we have the following:

Spells that create cones or lines of effect that originate from you also have a range of self, indicating that the origin point of the spell’s effect must be you (see “Areas of Effect” later in the this chapter).

Spirit Guardians has:

Range: Self (15-foot radius)

so it clearly fits into this category. The important rule here is on page 84 of the Player's Basic Rules (page 80 if you have an older version of the basic rules PDF), under the Areas of Effect section:

A spell’s effect expands in straight lines from the point of origin. If no unblocked straight line extends from the point of origin to a location within the area of effect, that location isn’t included in the spell’s area. To block one of these imaginary lines, an obstruction must provide total cover, as explained in chapter 9.

So if a creature has total cover from the point of origin, they're not affected by Spirit Guardians.

However, in this case, the targets can be affected if they're standing directly behind the arrow slit.

A target with three-quarters cover has a +5 bonus to AC and Dexterity saving throws. A target has threequarters 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.

So if the enemies are behind the arrow slits, they have three-quarters cover rather than total cover, and they can be affected by Spirit Guardians.

There is an additional potential issue with this strategy, but that was covered in this question.