If my party is near a fortress, can they stealth past it?

Question

I use D&D 5e at my table, and I want my party to sneak past a fortress of goblins behind arrow slits, and the arrow slits are shielding part of their vision. I'm a new Dungeon Master and I would like input for what I should set the DC for the stealth check to.

Answer 1

Stealth checks are typically checked against the Passive Perception score of the enemy.

The rules for "Noticing Other Creatures" in the Dungeon Master's Guide tell us (p. 243):

Outdoor visibility can be hampered by terrain, weather, and time of day. Creatures can be more likely to hear one another before they see anything.

If neither side is being stealthy, creatures automatically notice each other once they are within sight or hearing range of one another. Otherwise, compare the Dexterity (Stealth) check results of the creatures in the group that is hiding with the passive Wisdom (Perception) scores of the other group, as explained in the Player’s Handbook.

Passive scores are explained in Chapter 7 of the Player's Handbook (p. 175):

Here’s how to determine a character’s total for a passive check:

10 + all modifiers that normally apply to the check

If the character has advantage on the check, add 5. For disadvantage, subtract 5.

So if the party attempts to pass stealthily near the fortress and the guard keeping watch in the fortress has +4 to his Perception, then the group make a Dexterity (Stealth) check against the gaurd's passive Wisdom (Perception) score of 14.

In this situation, I would consider using a "Group Check", as explained in the PHB immediately following the section on Passive Checks (p. 175):

When a number of individuals are trying to accomplish something as a group, the DM might ask for a group ability check. In such a situation, the characters who are skilled at a particular task help cover those who aren’t.

To make a group ability check, everyone in the group makes the ability check. If at least half the group succeeds, the whole group succeeds. Otherwise, the group fails.

Group checks don’t come up very often, and they’re most useful when all the characters succeed or fail as a group.

So going back to our fortress scenario where the guard has a passive Perception of 14, you would have every party member roll a Dexterity (Stealth) check, and then if at least half of the party got a 14 or higher, the group succeeds, and if they do not, they are noticed by the watchman.

As a final note, if you believe the arrow slits should represent a hindrance to the guard's ability to notice the party, you might consider giving the guard disadvantage, which for a passive check is a -5 penalty (so for our guard gives a passive Perception of 9 instead of 14). Inadequate lighting can also confer disadvantage to the guard, as the rules for Vision and Light state (PHB, p. 183):

In a lightly obscured area, such as dim light, patchy fog, or moderate foliage, creatures have disadvantage on Wisdom (Perception) checks that rely on sight.

Dim Light is defined as (p. 183):

Dim light, also called shadows, creates a lightly obscured area. An area of dim light is usually a boundary between a source of bright light, such as a torch, and surrounding darkness. The soft light of twilight and dawn also counts as dim light. A particularly brilliant full moon might bathe the land in dim light.

Answer 2

Yes.

The stealth skill covers all uses of 'hiding' or 'sneaking'. If a creature cannot see or hear someone (due to distance or intervening walls) then there is no need for a Stealth check - they automatically are undetected. Simple line of sight or ability to hear doesn't automatically find someone who is Hiding. The creature in question must either make a Perception check or the creature's score in a Stealth roll compared to the creature's Passive Perception (for situations where rolling a lot of checks would bog down the game, or it is unimportant).

A very common new DM mistake is to assume that Hiding is impossible if an enemy has line of sight on an area. It may become harder, and a DM can ask for a higher DC (either a flat number or giving a bonus to the perception (passive or active) of the enemy) if the situation would make hiding more difficult (like the ground is covered in uneven trash that slides and bangs if you step on it wrong), but it should never be impossible or you disable the possibility of 'sneaking up behind someone', something that occurs in pretty much all fiction of any kind anywhere.

Often things that would impede Stealth are best treated as the reason someone fails to hide if they roll poorly - things underfoot or lines of sight or bad luck with guard patrols. Trying to map all of those out and having the roll automatically fail if they do creates a different game than the one D&D intends you to play, and is often unfair or requires a lot of reading the DM's mind.

Answer 3

Use the goblin's passive Perception score as the DC

By default, the rules for hiding (p. 177 PHB) state:

You can’t hide from a creature that can see you

In 5e, by default there is no facing in a combat situation (Facing is an optional rule in the DMG). That means by default, a creature in combat can see 360 degrees around them. So you never could sneak up to someone in plain sight in a combat situation, when they are alert and looking around.

But outside of combat, this should be possible. A guard may be mostly looking in one direction, bored by his dull task. Or you might be able to observe them, waiting until they look the other way to make your move. Or you may be hard to see because you are hidden in shadows, or behind soft cover.

The DMG says this on page 243 about Noticing Other Creatures outside:

Outdoor visibility can be hampered by terrain, weather, and time of day. Creatures can be more likely to hear one another before they see anything. If neither side is being stealthy, creatures automatically notice each other once they are within sight or hearing range ofone another. Otherwise, compare the Dexterity (Stealth) check results of the creatures in the group that is hiding with the passive Wisdom (Perception) scores of the other group, as explained in the Player's Handbook.

So if the player characters try to be stealthy, you would compare their Dexterity (Stealth) checks to the passive Wisdom (Perception) score of the goblin behind the arrow slit, which is given in their stat block: 9.

If the entire group is trying to sneak past, you can have them make a group check (p. 175 PHB):

To make a group ability check, everyone in the group makes the ability check. If at least half the group succeeds, the whole group succeeds.

Depending on the circumstances, you can also give the group's members advantage on their checks, or the guard disadavantage (-5 on a passive check, making their passive Perception score 4). Normally, I think an arrow slit would not do that -- it may limit the angle of view, but does not hinder how well you can see anything in the area you can observe, and if the PCs need to pass through that area, the limited angle of view is not much of a disadvantage.