Difficulty is a hidden mechanic in Nuclear Throne that the player can't see but affects the game in a few ways, so learning about how it works may help you in your runs.

Difficulty quite simply is a hidden number or stat that rises every time you enter a new level. Difficulty starts at `1` when you begin a new run and rises by one every time you enter a new level. So at 1-1 it's `1`, at 1-2 it's `2`, at 1-3 it's `3`, at 2-1 it's `4`, and it goes on like this forever.

One important detail about difficulty is that that entering secret levels raises the difficulty too, potentially increasing the difficulty more than usual:
- You enter 2-1 (4 difficulty)
- You enter 2-? (5)
- You enter 3-1 (6)

With this path the difficulty rises more than normal with inclusion of pizza sewers because it's an extra level visited, and it doesn't matter that it's not a part of the main route. Visiting secret areas that don't replace any normal levels increase the difficulty more than if you don't visit them. This is what difficulty would look like you *didn't* go to pizza sewers in your route:
- You enter 2-1 (4 difficulty)
- You enter 3-1 (5)

The same thing happens when you visit crown vaults. Going from 5-2 to ??? to 5-3 increases the difficulty more than going straight from 5-2 to 5-3. This even happens with IDPD HQ and YV's crib in some scenarios (more on it later).

Not all secret areas increase the difficulty more than the normal route though, one example is the jungle area. If you enter the jungle then you will go from 5-1 to 5-? and exit to 5-3 which doesn't increase the difficulty more than the normal route because jungle replaces one level (5-2) instead of being an additional one.

Secret areas that never increase difficulty:
- YV mansion (replaces 3-2)
- Cursed Crystal Caves (replaces 4-1)
- Jungle (replaces 5-2)

While you can gain difficulty by visiting extra levels you can also lose it by skipping levels with the oasis skip. Oasis skips several levels and reduces the number of levels that you enter and that impacts the difficulty number:
- You enter 1-1 (1 difficulty)
- You enter 1-? oasis (2)
- You enter 3-3 (3)

With the normal route you get 7 difficulty when you reach 3-3. By going to oasis from 1-1 you lose 4 difficulty compared to the normal route. The difficulty you lose varies depending on which desert level you entered oasis from or if you killed big bandit fast enough (you go to 2-1 instead of oasis if you didn't).

Weapon Drops

The primary effect of difficulty is weapon drops. The area that weapons can drop in is based on the current difficulty number rather than the current level directly.

For convenience we'll call the minimum difficulty required for the weapon to drop its *drop area* or *tier*. Let's take the hyper rifle as the example. Hyper rifle's drop area is `8` which is equivalent to the difficulty you reach after entering 4-1 with the normal route.

However the normal route doesn't matter, the only thing that matters is the current difficulty number as far as the game's concerned. As you already know the difficulty number can vary depending on the route, it can be higher or it can be lower. For example when you reach 3-3 you normally have `7` difficulty but if you visited the pizza sewers then the difficulty will be `8` which will allow the hyper rifle to drop from 3-3's enemies earlier than usual! The opposite effect is also true: if you lost difficulty by using the oasis skip then at 3-3 you will have only `3` difficulty which is not enough for hyper rifle to drop, even from a cursed chest (see the list below).

The difficulty required for weapons to drop slightly varies depending on where the weapon drops from. Here's what affects weapon drop tiers:
- Killed enemies' (such as big maggots) weapon drop tier is as high as the current difficulty.
- Normal chests' drop tier is as high as the current difficulty *plus* one. This means that on 1-1 with `1` difficulty you can find an assault rifle from a chest despite it having a drop tier of `2`. In the example above we said that hyper rifle's drop tier is reached on 4-1 but thanks to the +1 tier chests have it can drop on 3-3, just not from killed enemies.
- Big weapon chests have the same drop tier as normal chests.
- Cursed chests' drop tier is the current difficulty plus *three*. For example a cursed chest on 1-1 can drop a triple machinegun which has a tier of `4` while the current difficulty is `1`. Cursed chests can never drop weapons which have a tier of `2` or lower. For example assault rifle (`2`) or wrench (`1`) can't drop.
- Big cursed chests from cursed caves have a drop tier as high as the current difficulty plus *four*. The minimum tier of its drops is also higher than normal cursed chests, never dropping a weapon with a tier equal to `3` or lower.
- Giant weapon chests' (from YV's crib) drop tier is as high as the current difficulty plus *six*. It doesn't have a minimum tier for its drops, so it can drop anything.
- Robot gets additional +1 to drop tier no matter where the weapon drops from. This stacks with tier bonuses that chests have, for example a cursed chest has +3 tier and robot has +1, together it turns into +4.

Note: there is a belief that normal chests have the normal drop tiers while killed enemy drops are -1. This is technically less correct but it doesn't make much difference.

Here's the drop table of all weapons: (imagehere)

Enemy Number

The second thing that difficulty affects is how many enemies level gen spawns. Generally, higher difficulty - more enemies.

Difficulty affects two things in level gen: minimal number of enemies on the level and the chance of enemies to spawn for each floor tile. Short summary of how level gen works regarding enemy spawning:
1. The game places floors to form the level layout. The number of floors is 118 for every area on average.
2. The game runs through every floor and spawns an enemy with `difficulty/(difficulty+10)` chance.
3. The game runs through every floor again and spawns enemies until the current number of enemies spawned is equal to or higher than `3+(difficulty/1.5)` or until it runs out of floors.
4. If Crown of Blood is being used then repeat step 2 but with `difficulty/(difficulty+8)` chance of spawning.
5. In loops run through every enemy spawned by previous steps and with `loop/60` chance duplicate the enemy `3+loop` times.

The rest of this section will be a more detailed explanation of the steps listed above.

The first step is not much of an interest for our topic. Level gen spawns floors until the total number of floors spawned reaches a certain goal which is 118 between all areas on average.

The second step is one of the steps that spawn enemies, we'll call them *spawner* steps. The way it works is go through every single floor that level gen spawned and do certain checks on each one of them individually. These checks are:
- Is this floor far enough from the player's spawn?
- Is this floor not occupied by a chest or rad canister?
- Does RNG roll in the right way? (the chance is `difficulty` in `difficulty+10`, e.g. 1/11, 2/12, 3/13, etc)
If all the checks pass then the game spawns some enemy on the current floor. The game moves on to the next floor tile and this process is repeated until it's done with every floor tile. It also should be noted that extra difficulty has less and less effect the higher it already is, the chance of spawning enemies gets closer to 100% with each difficulty point added but never actually reaches it.

The third step is kind of similar to the second step but it's different. It's the "min-step" that ensures that there are enough enemies on the level so it can never be empty. It works fairly similarly with one difference. Again, it goes through every floor tile and makes checks:
- Far enough?
- Free from chests and rad canisters?
- Is the total number of enemies spawned lower than `3+(diff/1.5)`?

If all checks passed then it spawns an enemy on the current floor and moves on to other floors, again just like step 2. Spawner steps are not concerned about placing enemies on top of each other, two spawner steps can spawn two enemies on the same floor tile just fine. The third check is the difference and why it's called the "min-step". The minimum number of enemies is `3+(diff/1.5)` and the min-step ensures that there are at least as many enemies. However if the difficulty gets very high then the min-step can't spawn enough enemies to reach the minimum number because spawner steps are limited to the number of floors on the level. When the difficulty gets that high the min-step practically spawns additional enemies on top of the second step.

The fourth step is done only if Crown of Blood is being used and it's also a spawner step. It's mostly identical to the second step with the only difference being the chance to spawn an enemy which is `difficulty/(difficulty+8)`. Crown of Blood practically doubles the number of enemies on every level when only step 2 and 4 are working, but as the difficulty grows higher (around the end of L6 or beginning of L7) the min-step starts to kick in and starts spawning lots of enemies as well which lessens the effect of CoB. Before that point CoB makes up for 1/2 (x2) of the kill count, when min-step kicks in it's only 1/3 (x1.5).

The fifth step, cluster spawns, is not directly affected by the difficulty. It goes through every single enemy spawned by previous steps and runs a random check to decide if the current enemy should be duplicated where the chance is `loops/60` (e.g. 1/60, 6/60, 15/60). If the random check was successful then the game duplicates the enemy `3+loops` times. Just like spawner steps it repeats this process for every single enemy.

So in short, difficulty increases the chance of enemies spawning on every floor and the amount of enemies spawned by the min-step. The more difficulty you already have the less effect gaining more has due to how the spawn chances are written.

HQ Abuse

HQ abuse means making use of the HQ in order to increase difficulty by entering and leaving it.

Let's start with the most uncommon and inefficient way to do HQ abuse on X-1 levels when HQ replaces a level with itself, just like jungle:
- You enter 3-1 L1
- You enter HQ1
- You enter 3-3 L1

In that case the difficulty is unchanged. You can wait until HQ2 before leaving and still exit to 3-3 L1 which would give you one extra difficulty but this way of doing HQ abuse is never used. 1-1, 3-1, 5-1 and 7-1 are the only levels where HQ acts like jungle and replaces a level; everywhere else HQ is an additional level and in some cases it even repeats the level you entered it from. For example a better way to perform HQ abuse:

- You enter 2-1 L1
- You enter HQ1
- You enter 3-1 L1

No levels were replaced by HQ so you gain difficulty from doing this. HQ never replaces a level in any case other than the previous example. Again, you can also wait until HQ2 before leaving to gain +2 difficulty instead of just +1.

But the most optimal and common way to do HQ abuse is to use levels that HQ repeats upon exit. For whatever reason if you enter HQ from a boss level of an open area, such as 1-3, 3-3, and 5-3, you will exit at the same level you entered from. For example:
- You enter 1-3 L1
- You enter HQ1
- You enter 1-3 L1

In openbeta and NTT v100 exiting HQ doesn't repeat levels. Instead of 1-3 you will be sent to 2-1 which still gives you more difficulty than the normal route, just not as much as non-beta. Repeating levels works in non-beta, NTT 9940 and NTT 9944.

Whenever you perform HQ abuse you always have the option to either exit from HQ1 or HQ2. Leaving from HQ1 is less risky because if you fail to exit then you get another chance at HQ2. Leaving from HQ2 increases the difficulty more but you have only one chance to escape; miss the van out once and then you will be forced to fight captain and end the run there.

Optimal HQ abuse can be performed from 1-3, 3-3 or 5-3. On versions where level repeating is fixed it can be done anywhere other than X-1 on open areas. It can also be done on L2 which some players prefer. HQ abuse can be done only once per run, as trying to enter HQ again after leaving it once will blow up any van that you attempt to interact with. On L3+ HQ abuse is no longer possible.

YV Crib Oddities

(all the info is untested and I have no idea if all of this is different in different version)

YV crib is the special level that can only be accessed by YV when he reaches level ultra. When he picks any of the two of his ultras he will be sent to the crib. This may or may not skip levels depending on where he got level ultra.

One of the most common levels to get level ultra from is 7-3, where you get rads dropped from the throne and get level ultra from them. Then you get an ultra which sends you to the crib and returns you not to 0-1 L1 but to 1-1 L1. This is one of the cases where crib skips a level which are the most common.

One of the cases where crib doesn't skip a level is with boss levels. For example:
- You enter 1-2 L1
- You enter $$$
- You enter 1-3 L1

If this happens then you get +1 difficulty like from an extra pizza. This also happens preloop if you managed to reach level ultra on 7-2. You will be sent to crib and brought back to 7-3 with no levels skipped.

If you charge up and open a vault on a level and also get level ultra on the same level then after leaving crib you will not go into the vault but back to the same level where this vault was repeating it. It doesn't count as visiting a vault, so if you have already visited two vaults and you got ultra while entering the third vault it will still count as if you visited only 2. This is essentially the same as doing single HQ abuse which gives +2 difficulty.

YV crib can also skip several levels and reduce difficulty like oasis if you get ultra before labs preloop. Usually it happens when you start with crown of blood and go to cursed crystal caves which have a very high amount of rads, often enough to reach level ultra. If you reach ultra so early then you will be sent straight to 6-1 potentially skipping the entire frozen city if you ultra'd right in cursed caves:
- You enter 4-?
- You enter $$$
- You enter 6-1

If you reach ultra on 5-2 and open the vault portal there then 5-2 will still be repeated.

Summary - Is it worth it?

If people hunt for extra difficulty it's for two things:
- Better weapons early
- Extra kills (relevant for dailies)

Both of these things are correct but sometimes overestimated.

The first benefit of difficulty is making high tier weapons drop early. If you are an experienced player that gets to L1 and L2 consistently and survives there for a decent amount of time you probably don't need the extra difficulty. The highest drop tier of all weapons belongs to the ultra weapons which normally start dropping at 2-1 L1. When you get there the extra difficulty stops having an effect on weapon drops and because an experienced player can get to that point without trouble they can easily wait for high tier drops. This is not to say that extra difficulty is useless, having high tier weapons drop earlier simply can't be bad, the point is only that it's not very important.

If you are not an experienced player, are trying to unlock melting's B skin or playing hardmode, whatever the reason might be, the need for good weapons is more urgent, you can't wait for weapons, so having high tier weapons as soon as possible greatly helps with survival. Although difficulty doesn't make *that* much difference it still should be increased just for mere chance of a good weapon dropping early as it can make your run much easier.

The second benefit is higher enemy count which lets you have more kills. It is completely correct but also often overestimated. The amount of extra kills from higher difficulty is quite low and gets smaller and smaller as the run goes on because the higher the difficulty is the less effect adding more will have. Practice often shows that there's the difference between runs that go out of their way to do double HQ abuse and get every pizza possible and runs that don't do either is very small. Again, extra difficulty simply can't hurt and you *will* get more kills from it but the extra amount is negligible enough to ignore it.

Hardmode Difficulty

There are a few differences with how difficulty works in hardmode.
- Difficulty starts at `18` from 1-1 H1
- Every visited level increases the difficulty by two instead of one
- Cursed chests have +3 tier instead of +2
- Every time a weapon drops the game pretends that the difficulty is lower when deciding which weapon to drop. It works like this:
1. Drop tier is the current difficulty
2. Subtract `16` from the tier
3. Multiply the tier by 1/3 (0.33%)
4. Round the result up
5. Add `+1` to the tier
6. If there are any tier bonuses (chests and/or robot) add them last

Weapon tiers are exactly the same as in normal mode, the difference is only in hardmode's tier scaling.

Here's the weapon drop table adapted for hardmode: (imagehere)