1-4 players, no scaling (1 person doing a level and 4 people doing a level will get the same experience in terms of enemy density, items, etc). You can duo or solo if you wish, but it’s agonizingly hard on later missions.
Specifically stealth-based. The basic enemies - Sleepers - are sensitive to light and movement, but not vision. You can get right up next to them without them noticing, which is the ideal way to progress because a single charged melee swing will kill all enemies except the Giant.
If you move too quickly or while they are suspicious (represented by extremely loud gooshy heartbeats and the monster writhing around), they will aggro to you, and if you don’t stunlock or kill them right away, they will scream and aggro the whole room, which is a huge drain on your resources.
There are specific horde events - represented by “This will trigger an alarm!” - where enemies swarm your position from a specific direction. You can close and reinforce doors ala Killing Floor 2 to buy time, which is very useful because killing enemies during a horde only spawns more until the alarm is disabled.
There’s also a terminal system you can use to find items, and environmental awareness is super important. All level chunks are demarcated as “zones”, and info you get from terminals tells you which zones objectives are in. Plotting a route through the level on the fly is important, as is understanding the layout so you can retreat to more defensible positions.
I’m really enjoying it, despite the difficulty, because every time I play I feel like a cool sneaker hacker man, and I learn more about the nuances of the enemy AI. Like how the Sleeper AI design is intended to cause you to panic the moment you make a single mistake, but if you keep your cool after alerting a Sleeper, you can prevent the room aggro. It also has a huge focus on making the best out of a bad situation with limited resources, since you’re basically never topped off on ammo and the item distribution is relatively stingy unless you’re either sneaking or meleeing every enemy.
Really great game. The perfect level of tension in a co-op game.