Bring your own underwear.
I played this game after playing Outlast 1&2 and Evil Within 1&2. At first, I was disappointed, as the first hour is little more than scary noises, ghostly guff-clouds and an invisible force chucking furniture around - stuff I've already seen elsewhere. And your character freaks out in the dark, when the dark is the least of his concerns. (If anything, it's his friend, the kind of abusive friend who "protects" you by bullying you.) Not a recipe for 5-stardom.
But then.
BUT THEN.
The game takes its first turn when you realise there are monsters in there with you. They are deployed sparingly. They are absolutely terrifying. You can't fight back. You can only run and hide, listening to them growling for you, while off-kilter music plays and your character freaks out. By the time you reach the prison area after fixing a lift which takes you into the bowels of the castle, it's brown trousers time, ALL THE TIME.
You are given tinderboxes and a lantern to light your way. There are tons of tinderboxes, and tons of things to light, and light equals sanity. But the brighter things are, the easier it is for the monsters to find you. It's a balancing act. You must go into the dark, letting your character's eyes adjust, and risk your mental health to keep yourself alive.
Things get frustrating at times, particularly a nightmarish water sequence where you must hurry to escape without falling into the water, and sometimes items can be hard to find. Do not let this put you off. This game is a masterclass in agonising terror. There are only three games I ever turned off because I couldn't face my fear: Doom for the Sony Playstation (Aubrey Hodges' masterpiece soundtrack of babies crying and demons howling prayers), Evil Within 1 for its spider boss, and Amnesia, for... well, bloody everything.
Update 9 Feb 2021: The Justine expansion gets a hard fail from me. Unlike Amnesia, it's not really a game, just a short and maddening series of puzzles which are REALLY obscure, with scary moments thrown in. Even with a guide I struggled to find one or two documents and unlike in the main game, you need to rip books off bookases to find vital information. If you liked user-made classic Doom maps where secrets vital for progression were totally unmarked, you'll like this.