Downwell

Downwell is a minimalistic roguelike-platformer hybrid. It is very short and small, but has a surprisingly level of mechanical depth which makes it entertaining to play many times over. Given that the scope of the game is quite small and pretty much all mechanics have been fairly well-documented, this spoiler page will just contain various tips rather than an outright guide. There are plenty of guides available elsewhere on the internet.
The game is playable on both PC and iOS/Android, but I have found it is much easier with a keyboard than a touch screen. This is due to the control buttons being at the bottom of the screen on mobile, such that your fingers obscure upcoming enemies and obstacles more often than not.

Generally, you may choose one of three upgrades at the end of each level. If you are using boulder style, you only have two choices, and if you are using handstand style, you will not be offered upgrades at all. The "Youth" upgrade gives you an additional upgrade option after each level. I have listed the upgrades below and categorized them by usefulness.

Apple

Not useful. Restores 4 health. If you are at full health, it grants you an extra health container. Unless you are in low health, I don't recommend it, as health is not necessary if you don't take damage.

Blast Module

Useful. Causes an explosion beneath you when stomping on an enemy. Great for clearing out large amounts of enemies at once and also destroying collections of blocks simultaneously.

Candle

Not useful. Grants a longer amount of invincibility after taking damage. This upgrade is near-useless if you don't take damage often.

Drone

Conditionally useful. Spawns a friendly robot drone that shoots when you shoot. It follows near you and not exactly behind you, such that it gives you a little extra width on your bullet spread. I have found that it does not make a huge difference, especially when your primary method of killing enemies is stomping on them.

Gem Attractor

Very useful. Sucks in nearby gems. Maximizing your gem count is the best way to get the most value out of shops, so getting this early is extremely handy.

Gem Powered

Very useful. Refills ammo when picking up gems. This makes it much easier to build up a sizable combo for bonuses. It also makes Limbo easier, as Limbo doesn't have nearly as much to stomp on as the other worlds. This upgrade synergizes very well with Gem Attractor.

Gem Sick

Very useful. Increases duration of Gem High. Gem High increases weapon range and damage, both very useful attributes.

Gunpowder Blocks

Useful (especially in the early game). Blocks shoot a bullet upwards when destroyed. Additionally, destroying a block will cause a chain reaction which destroys all the blocks near it as well. This makes it easy to clear out large amounts of blocks which are preventing a smooth descent. It also usually kills most enemies above the blocks from the resulting bullets.

Heart Balloon

Not useful. Gives you a heart balloon which reduces your fall speed while you have it. If an enemy touches it, the balloon explodes, respawning once you complete the level. Given how easy it is for the balloon to pop, it doesn't provide much use past the first portion of any level.

Hot Casing

Useful. The discarded casing from shooting the gun boots deal damage. This upgrade is useful for dealing with large amounts of enemies which are not directly below you, particularly whne you have a gun which shoots a lot of bullets very quickly.

Knife and Fork

Very useful. Touching a corpse causes you to "eat" it. Eating ten corpses grants you +1 health. Not only is this upgrade a good way to stay in good health, it can often increase your maximum health. It is particularly useful in the Aquifer zone, in which corpses fall very slowly and are easy to catch up to while falling.

Laser Sight

Useful. Grants a laser sight beneath the player for aiming and also increases the range of the gun. The first benefit doesn't really do much, but the added range is actually quite useful for nearly all weapons other than the laser, which already has infinite range.

Member's Card

Useful. Grants a 10% discount at shops and increases the number of shops. This upgrade is obviously better the earlier you get it, especially if you are collecting a large number of gems.

Popping Gems

Useful. Getting a gem shoots a bullet upwards from the player. This upgrade becomes more useful later in the game when there are more enemies coming from above. This also synergizes very well with Gem Attractor, as you will be shooting a lot more bullets upward from all the gems attracted.

Rest in Pieces

Useful. Shooting corpses causes them to explode, damaging nearby enemies. This upgrade is great for clearing out large numbers of enemies in groups. Note that it has a negative synergy with Knife and Fork, as shooting the corpse makes it impossible to consume it.

Reverse Engineering

Not useful. Allows you reroll each gun module once by shooting it. None of the weapons are bad enough to warrant rerolling, unless you are really trying to stick to your favorite weapons.

Rocket Jump

Not useful. Creates an explosion when jumping from solid ground. Given that it is not usually good practice to spend a lot of time on the ground in Downwell, this upgrade is not very useful.

Safety Jetpack

Useful. When out of bullets, allows the player to hover for a short time. Recharges when landing on stomping on enemies. This can allow for some additional time needed to keep up a combo or move out of danger.

Timeout

Not useful. Creates a space where time freezes whenever you get hit. Not only is this upgrade made entirely useless by good dodging, it is actually quite difficult to remain in the time void once it is created.

Youth

Useful. Gives you +1 health and an additional upgrade option to choose from at the end of each level, for the remainder of the run. Note that the usefulness of this upgrade falls off drastically the further you get in the run, as you have less chances to actually make use of the additional upgrade option.

Styles are permanent modifiers chosen at the beginning of a run which are in effect until the end of the run. You cannot change your style mid-run.

Arm Spin Style

Eliminates gem veins from side rooms such that only gun modules appear. Shops also become more rare. This style is useful for when you are not that interested in making use of gems, preferring to maximize your firepower and health through upgrades and gun modules.

Boulder Style

Increases your starting health to 6 and makes you fall slightly faster. Additionally, it lowers the number of upgrade options you get at the end of each level to two, but this can still be increased by the Youth ugprade. This style is useful if you need the health or just want to go fast, but otherwise, dodging is always preferable to increased health.

Handstand Style

Grants a 30% discount at all shops, but you are offered no upgrade options at the end of a level. Given that upgrades are usually vital to the success of a run, this style is pretty bad.

Levitate Style

Decreases your falling speed, as if you had the Heart Balloon upgrade. This style is very useful for building up large combos, as the increased hangtime gives you more time to maneuver to avoid landing prematurely.

Usual Style

The standard, starting style. You start the game with no special modifiers.

There are four worlds in Downwell: Caverns, Catacombs, Aquifer, and Limbo. Each world contains unique enemies and other features.

World 1: Caverns

The Caverns are mainly full of weak enemies and groups of blocks. This are is good for getting a lot of gems quickly from combos, as it is relatively safe compared to other worlds.

World 2: Catacombs

The Catacombs are full of ghosts and skeleton enemies. Beware of differently-colored floors, as spikes will pop out of them shortly after landing on them. There are also areas entirely blocked by groups of destructible blocks, which makes the Gunpowder Blocks upgrade very useful in this world. You will have to be a little more careful where you shoot in this world, as floating skulls will become enraged when shot and follow you. Also, the large ghosts are very tanky and take a lot of bullets to kill, so I recommend just stomping on them whenever possible as that instantly kills them.

World 3: Aquifer

The Aquifer world takes place entirely underwater, meaning that you have to constantly break debris and shells in order to consume bubbles for air. If your air meter drops to 0, you will begin to take damage very quickly. Note that in Hard Mode your air meter will run out much faster and you will have to collect single bubbles as you fall. Due to the water, corpses in this world fall much slower than in the other worlds, making it easier to catch up with them and consume them with the Fork and Knife upgrade.

World 4: Limbo

In Limbo, there are very few platforms and you must kill the phantom enemies by shooting at them rather than stomping, as they deal damage upon direct contact. Limbo is a great place to rack up large combos since there is very little solid ground to land on and cash in. Note that even in side areas where it appears like there is no ledge to cash in a combo, walking off the ledge from the time bubble will indeed trigger the combo to end, allowing you to start a new combo whenever you see a side area.