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The HUD

So, from left to right, roughly:

We've got my name and my current health. Note that characters have 50 max HP and generally don't increase that value any further, so divide it by 2 to get my actual HP value.

"tired" means that I don't have running mode ready. Cautious means that I haven't used up my run for this level and running means I'm in the middle of doing so. Healing or walking down stairs changes this from whatever it is to cautious. I talk about it in the video, but running makes me fast and dodgy with a penalty to my accuracy.

Exp is presented like it is in a lot of roguelikes. I am level 3, I am 86% of the way from 3 to 4.
My level doesn't have any strange, invisible effects on anything. I choose a trait before each level up and that's the only way my character becomes more powerful.

My armor protects me from a single point of damage every time I get hit, which is the maximum level of protection that green armor can give - hence 1 out of 1. The 52% is its durability, it's 48% of the way to falling apart.

Armor also gives penalties to move speed and being knocked back, but it's hardly noticable on anything less than red or very heavy special armor. Boots make you take less knockback as well, but they have no movespeed penalty.

My weapon deals 8d3 damage (that is, 8 rolls of a 3 sided die, so about 16 average) and is currently loaded with one shell, which is the most ammo I can put in to it. One shotgun mechanic that I don't really mention is that they have a cone-shaped spread that deals full* damage to all enemies - they don't provide eachother cover like in Doom.
*Well, there's damage falloff per tile, which is applied to each enemy individually, based on their range from me.

The level name in the bottom right tells me where I am. If it's red, there's still enemies around to kill.

I don't use the minimap very often. The colours I do look for are green (myself), yellow (stairs) and red (enemies).

Resistances
All armors provide some resistances. I'll generally discuss any special armors I get, but the basic armors give the following resistances (at full durability - resistances degrade as the armor does)
Green: 10% to bullet and shrapnel
Blue: 15% to plasma
Red: 20% to fire

You can get resistances from other places too, I'll explain those in the video. Note that resistances are added together, not multiplied. They cap at 95%, but at that point you're taking one damage from every instance of that damage so I don't think you care.

Sidestepping
Each enemy has a chance to miss you if you dodge to the side just before they shoot. The chance is larger for some enemies, mainly early-game enemies that don't use guns like the imp and cacodemon.

Some enemies either can't miss (only shotgunners) or have ground-targetted AoE attacks (revenant, arch-vile) that don't respond well to stepping out of the way.

If you dodge the wrong way then they might target the tile you were in one turn ago, but hit you anyway because you didn't actually leave their firing line. That's why it's called "sidestepping" and not "towardsthemstepping".

Mods
There are 4 types of mods. I use a lot of mods, but I only explain a little bit why I'm using them on various items.
Agility Mod: Gives movespeed on armor. Gives accuracy on weapons.
Bulk Mod: Gives durability (but also a larger movespeed penalty!) on armor. Gives ammo capacity to weapons.
Power Mod: Gives protection value to armor. Gives damage to weapons.
Tech Mod: Gives knockback resistance to armor. Gives firing speed to weapons.

So, which items get what?

Boots: Always agility. I'm not wearing these because I plan on stepping in lava, I'm basically wearing them as a socket for an agility mod.

Armor: Often agility, but any non-tech mod has its usefulness. Bulk/power red can get you through tough spots thanks to the fire resistance keeping you safe from most end-game enemies.

My shotguns: I'm mainly thinking about assemblies here. While most items can only be modded once, some two-mod combos will work and give you an item with a new functionality. The ones I like most for shotguns are:

Elephant gun - Regular shotgun + P + P. Deals massive damage, has massive reload time. I feel that if you think you want a double shotgun then you probably actually want this, because it has the same drawbacks but with better damage falloff and better armor penetration (that is, it deals 12d3 damage instead of two lots of 8d3).

Tactical shotgun - Combat shotgun + T + P. Can fire on consequtive turns, unlike combat shotgun. Excellent for any situation.

Foucsed double shotgun - Double shotgun + A + T + P. Woah! 3 mods! That means you need a level of Whizkid to make this work. This is the dream shotgun, great for everything except armor penetration. It does take a bit of doing to make it, though.

Rocket launcher: Any mod will work. They're all fairly average, honestly. Power mods are a bit overkill, tech mods only give +10% firing speed, bulk mods (give reload speed on one-shot weapons) don't mean much when I can reload on the move anyway, agility mods can be replaced by trick shots (aim at walls or multiple enemies to make hitting basically a certainty)

Plasma weaponry: A different kind of character could pull off some bullshit with a heavily-modded plasma rifle, but all I want is a bulk mod so that I can hold more plasma in less inventory space.

Chainsaw: A bulk mod. Power mods specifically increase the number that helps the least (eg. shotguns go from 8d3 to 9d3 and chainguns go from 1d6 x4 to 1d7 x4), so a power mod on a chainsaw makes it 4d7. Bulk mods make them 5d6 instead, which gives more damage on average. While agility would be nice to improve the 50-50 odds of hitting anything, any character that's actually planning on using their chainsaw will handle that by putting points in the melee damage/accuracy trait.