This game does not have a single game mechanic as core gameplay.
Usually games have one core mechanic, but this game just offers a bunch mechanics to use.
The idea is that experiencing the game world and learning the game mechanics is the core gameplay.
The game loop supposed to be:
1. Encounter a new mechanic
2. examine how it works
3. master the mechanic
4. move on
The more mechanics you have mastered the easier the game will be.
Easy mode should require no prior knowledge
Hard mode should be hard to beat with full meta knowledge.
For that to work each game mechanic must have a limited effect on the game.
Any mechanic that dominates the game should be reduced in effect.
Here is a list of actual game mechanics you'll encounter:
The most visible game mechanic is moving around using the wasd keys.
In combat movement it allows to evade enemies.
There are obstacles to avoid and to use by the player.
Out of combat, movement is mostly busy work, but can be skiped.
In tight timings you might want to optimise walking paths.
This is pretty barbones to be honest.
Bump into enemies to attack them. Enemies counterattack. People get hurt.
The damage, health, armor in combat depends on creature type, but is not random.
Healing should be relatively rare and enemies should hit hard.
So you should have to keep an eye on your stock of health vials.
A single enemy might not be able to kill you, but it still takes health points.
That will accumulate and kill you, if you don't mange your health.
To strengthen this, health vials encourage the player to waste them.
They heal less the more health a player has.
So a full heal from death takes a lot more resources than 2 heals from near death to half health.
Enemies can be kited. This means the player can run away from enemies and the enemies will follow.
They usually are lead into traps and/or killed one by one.
Within a tile the enemies can be blocked by obstacles and lured into traps.
When leaving a tile, enemies have a chance of chasing the player or staying there.
This border is clearly visible and that is offering the player to kite enemies.
I love kiting, but kiting is limited in the game. It would be too powerful otherwise.
Only a part of the enemies in a group can be kited in a short time frame.
So the player can split that group once and prepare, but cannot repeat that.
If you still want to kite a group of enemies multiple times, you can wait a longer time and the enmies can be kited again.
To be honest: this is my favourite gameplay mechanism ^^
Your equipment makes a lot of difference on your combat capability.
So one game mechanic is finding and equiping good equipment
To be honest here: Currently this is way to simplistic and utterly boring.
Effectively you get max stat equipment and are done with that game mechanic.
There is a bit of base management in the game. It mostly is ensuring that trap rooms are loaded.
You can reload trap rooms and NPCs will do that for you, too.
That takes a while, so kiting enemies into the base defences too fast can reduces charges to zero.
In that case the base will be overrun. So the player has to keep an eye on those trap room charges.
Enemies have behaviour logic that can be understood and is not strongly random.
This allow to watch what the enemies are doing and predict their behaviour.
This gives a significant advantage when dealing with enemies.
It is possible to combine landmines with bombs to trigger chain reactions.
Walls can be set up to change enemy movements and funnel them to specific places.
This allows to set up relatively complex multi part traps.
That whole thing is mostly a byproduct of the simulated world, but pretty fun.
I have some trap designs, but they were ingame with the available stuff without designing for it.
Other players might set up their traps in completely different ways.
The player needs to be promoted in the early stage of the game.
To be promoted the player needs to collect reputation.
So the player has to gather reputation.
There are multiple ways to that and the player can earn and loose reputation.
The player can optimise that to gain the needed reputation.