This review works on the assumptions present in the three core rulebooks. It does not consider various character options and spells available in other books (e.g., a character with a built-in swim speed or flying speed from level 1).
This review does not (directly) address light and visibility, avoiding hazards and traps, navigating and getting lost, finding specific objects or places, and so forth, although it may touch on these subjects.
Other abilities, spells, and tools not explicitly identified here could be used in novel or surprising ways to overcome barriers and obstructions (the classic wizard epiphany: “fireball is an exploration tool… fireball is a negotiation tactic!”). The following is a non-exhaustive and subjective review of character mechanics focused primarily for these purposes.
This review is primarily applicable to dungeon exploration, but some of the ideas can be extrapolated to other forms of play.
This is a non-exhaustive review, and I welcome additions to this list.
Soft Barriers and Hard Barriers
Soft barriers tax resources, demarcate dungeon zones, provide staging grounds for encounters, or simply prompt the players to make a choice in how they are positioning themselves against the dungeon as an environment, and the factions within it.
Hard barriers are meant to stop progression until the characters either have some specific means of gaining access (e.g., a key for a locked door) or have found a solution within their own abilities to subvert or bypass the barrier. It is important to note that hard barriers are not impenetrable barriers; they can almost always be undermined by clever or daring players.
Tier 1 (levels 1-4)
Character Abilities and Skills: A high-dexterity character proficient in Thieves Tools (+5), possibly a rogue with Expertise (+7); a high-strength character (+3); and a high-Wisdom character proficient in Perception (+6)
Spells (level 1-2): Alter Self, Feather Fall, Jump, Knock, Levitate, Misty Step, Spider Climb
Magic Items (mostly uncommon): Potion of Climbing, Potion of Waterbreathing, Cap of Water Breathing, Cloak of the Manta Ray, Mariner’s Armor, Ring of Swimming, Rope of Climbing, Wand of Secrets
Soft Barriers
Locked, stuck, and barred wooden doors. It’s important to understand that most normal doors are only soft barriers, even at level 1. For example, when players encounter a locked door, they will pretty reliably follow an order of operations like this, trying each step:
Unlock with the key: No downside.
If they don’t have the key, pick the lock: Probably no downside, but time-consuming,1 risks triggering traps associated with the lock, and close inspection later might reveal the characters’ meddling.
If no one is proficient with thieves tools, or picking the lock fails, force the door: Momentarily loud, risks triggering traps associated with the doorway generally, and obvious to anyone who sees it after the fact.
If all else fails, destroy the door: Sustained noise, risks triggering traps, and obvious to anyone who sees even the vicinity of the door after the fact.
A high-dexterity character with approximately a +5 on related Dexterity checks (proficiency in Thieves Tools) can pick a DC 10 to DC 15 lock fairly reliably; make that +7 for a rogue with Expertise. Failing that, a character with approximately a +3 to a Strength check to smash down the door (assuming no proficiency applies) presents a reasonable fallback option. Failing that, with something like AC 15 and 18 HP, even a group of first level characters can reliably destroy a door in one combat round.
There are consequences to each step along this escalation. But the point is that ordinary wooden doors – even locked or stuck ones – are soft barriers only.
The Angry DM’s megadungeon posts are instructive for setting DCs for doors along these lines. As they note, a DC 25 stuck wooden door presents an interesting complication; Tier 1 characters would either have to explore elsewhere or commit to physically destroying the door.
Secret doors. Unless combined with other mechanisms, a secret door on its own is a soft barrier only. Using 5E’s passive perception rules almost ensures easy and medium difficulty doors will be found, as at least one player in any given group will field a high-perception character.
Eschewing the passive perception rules and going with a more old-school approach where the character interprets clues from the DM’s description, like recently-applied plaster or a weird draft, doesn’t change the permeability of the barrier here. Assume secret doors are soft barriers only, even at Tier 1.
Ordinary bodies of water. All characters can swim, and even a character without a positive Constitution modifier can hold their breath for a minute. Water on its own is just a soft barrier.
Short climbs (the 50’ rule). Expect that most groups will sensibly secure a rope (using a grappling hook or a lead climber, if ascending) and that even in a worse-case scenario (weak climber falls, maybe cascading to others if they were tied together, and so on) the fall damage is unlikely to kill anyone.
The adventurer’s standard 50’ rope is a good rule of thumb. If the climb is less than 50’, treat it as a soft barrier that can be resolved with a single check for each climber (with advantage if they have a secured rope to climb with).
Short gaps. 10 or 15 feet is probably trivial with standard dungeoneering equipment and reasonable ingenuity from the characters. Treat that as a soft barrier.
Other dungeon obstructions. Quicksand or swampy ground, tar, tangled vines and thick undergrowth, dripping green slime, spider webbing, thin/porous walls of conventional dirt and rocks, and so forth. Careful, lucky, stubborn, or diligent characters will come up with ways to get through obstructions like these.
Hard Barriers
Stuck or barred metal or stone doors; metal portcullises. What would make some doors hard barriers at Tier 1, compared to the wooden doors described above as soft barriers? Removing “destroy the door” as the option of last resort.
As described above, destroying a wooden door, using the assumptions in the DMG about AC and damage, probably only takes characters one or two combat rounds. Destroying a stone or metal door should be described to players not as something that can be done in six to twelve seconds by a determined barbarian but more a matter of mining or demolition work.2
You can model these “hard barrier” doors with damage thresholds and damage immunities if you want to spell it out under 5E’s rules (see DMG p. 247). I would consider making such doors immune to all damage types except bludgeoning, force, and thunder, and setting a fairly high damage threshold.
Merely locked metal or stone doors are just as susceptible to thieves' tools as wooden ones, but it’s reasonable to assume that a room that warranted a steel door also got the best lock available; so if other locks on nearby wooden doors are medium difficulty, stone or metal doors should have hard or very hard locks.
I have not recently used a portcullis, which seems like a mistake, as they’re flavorful and hard to bypass, but provide useful information (because characters can see through them). Misty Step can circumvent them, assuming the stepper can find the winch on the other side, but spell slots are scarce at Tier 1, and I am OK with a player giving up a powerful combat trick to break a hard barrier in this manner.
Locked doors with Arcane Lock. This is a special case. It raises the DC to break, force open, or pick the lock by 10. Boosting a DC 15 door to DC 25 likely puts it out of reach of a Tier 1 character’s Strength check (if no skill applies). It would be within the range of possibility of thieves' tools, but would require a very good roll, with our previous assumptions of a +5 check, or +7 with expertise. Knock, a 2nd level spell, suppresses Arcane Lock, but only for 10 minutes, which creates interesting time pressure and the possibility that characters become trapped on the other side.
Arcane Lock doesn’t prevent the destruction of the door, but we can assume that if a dungeon faction is investing the spell components in protecting a door, they’ll make sure it is metal, stone, or otherwise hard to bypass (e.g., several Arcane Locked doors in sequence).
Note that Arcane Lock lasts until dispelled and has a modest material cost of 25 GP per casting. Depending on the arcane capabilities of local factions and the prevalence of magic in the campaign world, characters should expect to see Arcane Lock quite often, even at Tier 1.
Dirt and stone walls. This might seem obvious, but dirt and stone walls and floors of more than trivial thickness should be hard barriers. Attempts to mine or excavate require that the dungeon level is secure.2
The important exception to this rule is the druid, who at 2nd level can transform into a giant badger with a 10’ burrow speed. Note that 5E creatures with burrowing speeds can move through “sand, earth, mud, or ice.” At combat speed, a giant badger druid could burrow about 200 feet per minute; fast enough to navigate pretty quickly throughout the vicinity.
Within earthen areas like the tunnels of a giant ant colony, the DM should have some idea of where (if anywhere) some burrow-blocking material (usually layers of stone) exist. Likewise for a pyramid partially flooded with sand.
Water with complications. There are many spells and magic items at low levels that allow characters to breathe or move freely underwater, but most of them only do one or the other, and affect only one character at a time.
Visibility: Visibility is not usually a limitation within 5E, because of the prevalence of races with darkvision, and the availability of Light as a cantrip. But combined with murky conditions underwater, it can be a complicating factor, particularly if characters are considering swimming through a submerged tunnel without knowing how far it is to the other end.
Monsters: How much of a danger this represents depends on party composition, spells available, and the number of weapon-oriented characters wielding weapons unaffected by submersion (e.g. daggers, spears). A monster that acts more like a regional hazard may be helpful for turning water into a barrier; the characters might be tempted to try to fight crocodiles, but swarms of countless piranhas require a non-combat solution.
Temperature: For hot water, keep in mind that a reasonable number of characters will have fire resistance (tieflings and many dragonborn, for example). For cold water, some mix of cold damage and exhaustion may be appropriate. As is always the case with water, if the characters cannot easily gauge how far they have to go to get through the intemperate area, tanking the damage and conditions becomes less feasible.
Pressure changes: A diver can go something like 130’ without suffering from any decompression effects. Decompression could be simulated with Constitution saving throws to avoid levels of exhaustion. Pressure can be a good way to either gate a lower level of the dungeon (reachable by diving and accessing an airlock, moonpool, permeable magical wall, etc.) For navigating laterally within a dungeon level, this could be useful for an instance in which the characters need to swim under a dividing wall that requires they rise up 130’ or more on the opposite side. Magical effects that grant swim speeds should usually grant advantage on saving throws to resist decompression effects, depending on the flavor of the spell or effect.
Long climbs (beyond 50’). The difference between a short climb and a long climb is that the former can be resolved with a single ability check for each climber, while the latter is more difficult or elaborate. Even absent any other complications, a skill challenge or similar resolution mechanism is appropriate for a long climb. The number and types of ability checks, accordant DCs, and likely outcome of falling should be clearly telegraphed by the DM. The players need to be able to make an informed decision to either climb now, or explore elsewhere and come back later.
Long gaps (beyond 20’). A character with 18 strength can jump 18 feet; a character with Misty Step can go 30 feet. Somewhere in this vicinity, a gap turns from a soft barrier to a hard one for Tier 1 characters. Various schemes involving grappling hooks, familiars, and so forth can certainly break this barrier, but assuming a long fall (either deadly or dropping a character into a deeper part of the dungeon), it’s safe to consider this a hard obstacle.
Note the distance from the ground on one or both sides of the gap to the ceiling (if there is one) and the walls can be highly relevant here if the group has climbers, particularly the spell Spider Climb, which is a powerful sequence-breaker when used to rig ropes or otherwise facilitate the other characters’ passage.
Sometimes a hard barrier is interesting not just in how difficult it might be to pass over, but how hard it might be to employ the same means to come back the other way. A jury-rigged rope bridge across a 30’ gap may not be so practical when the characters are running in the other direction, chased by angry cultists and carrying stolen treasure.
Cave-ins, collapses, and blockages. A completely collapsed tunnel or passageway should be an almost-full stop barrier for a Tier 1 group, short of actual mining and excavation work.2
Extreme temperature, poison gas, and other pervasive environmental conditions. Like water temperature or pressure dangers, the obstacle is usually a hard one if the players can’t determine how far they would have to travel through the affected area, and can’t come up with a plan to avoid or tank the effects.
Damaging magical walls and Blade Barrier. Not all of the various wall spells detailed in the PHB include rules for making them permanent, but the Obstacles table on page 297 of the DMG makes it clear that it’s possible (even if it didn’t, this would be fair game for the DM to incorporate anyway, but it’s always helpful to note the precedent when it’s already in the game’s text). Walls of Thorns, Wall of Ice, and Wall of Fire all do varying amounts of damage to those who pass through them. A DM deploying these barriers should think about the effectiveness and consequences of the first one or two obvious countermeasures the characters might employ (what happens if they try to burn the thorns? What happens if they flood the passage blocked by the fiery wall?)
Next: Barriers and Obstructions, Tier 2
1. Note that a rogue with the thief archetype and the Fast Hands ability can open a lock as a bonus action. The thief archetype can sometimes appear lackluster compared to other roguish archetypes, but correct application of dungeon time-keeping and enforcement of consequences makes their abilities shine; the thief rogue should be able to attempt to pick a lock without risking time-related complications that any other character would face.
2. Persistent characters will tunnel through iron with toothpicks if allowed enough time to do so without complication. DMs should explain that mining, excavation, and demolition in unsecured areas is not something that can be done as long as any factions that might object are active on the dungeon floor. This provides an incentive to “beat” the current dungeon floor – befriend, defeat, pacify, or otherwise obviate all other factions on the floor. After that has been done, any remaining hard barriers of this kind can be overcome simply by sending some hirelings to manually destroy the obstruction.
