A Simple guide to Amnesia

First things first in Amnesia if you didn't already know there isn't any type of combat in this game, meaning you can't fight the enemies in the game. Secondly do NOT waste tinderboxes or lantern fuel on useless stuff, like lighting a candle next to another lit one, this isn't The Legend of Zelda where lighting up every torch you see gives you an item. Third you can always tell if a monster see's you if you here the terror music, which sounds just like below (not my video) and can tell if a monster is still around if you can obviously hear it groaning or walking, but an easier indicator is the music, when the music starts sounding creepy you know a monster is lurking around somewhere, just wait paciently in a closet or hiding behind something (not a door) until the regular music of the area plays. Always pay attention to your sanity, if it is low, (i.e if it says ... or heart is pounding, hands are shaking when your mouse hovers over you brain in the tab menu) then you might start hallucinating monsters. Expert advice is that to not use tinderboxes in big areas or long corridors, because you never know if those lit candles could mean the death of you. Meaning that you never know if the room you are in has monsters in it or not.
Good real life advice on this is to take breaks every once and a while, especially if you are young. Also avoid playing this at night unless you really want it atmospheric and if so, be my guest. Though I haven't experienced it, you might get nightmares if you play this right before you go to sleep and keep in mind Frictional Games aren't responsible if you wet your bed after playing.

Though there aren't that many kinds of monsters roaming around the castle, they should all be considered a major threat and shouldn't be taken lightly. The first and most common type of monsters are called The Gatherers which fall into two types, the Servant Grunt, and the Brute
(Note: These pictures on this page aren't mine and belong to their respective brave owners)

The Servant Grunt is one of the most common enemy you will encounter and will also be one of the weirdest. Though there are speculations on what might have created the Servant Grunt, there is evidence in The Wine cellar area of the game that Alexander, the main "antagonist" of the game drugged some of his worker's wine and locked them in the cellar to rot while the "wine" killed them and supposedly turned them into what is the Servant Grunt. The best ways to deal with them is to find a closet, small room, or dark area to hide by to prevent them from finding you, as they can usually 2-shot you or one shot you if you aren't careful. Most of them only come at certain times in the game and after awhile will leave the area. (Exception is near the end of the game)
The Brute is the other type of Gatherer that is more rare then the Servant Grunt and doesn't appear in many areas of the game. The only areas that the Brute can be seen in are the prison, the morgue, the Cancel, (last visit) and the Choir. There has been a lot of spectulation on what "made" the Brute, but the main two is one, that in the Wine Cellar where Wilheim and his man died thanks to tainted wine that Wilheim, when he died, turned into the brute and combined with Alexander gving the Gatherers weapon arms made the Brute and that only one brute exists, the other suggests that all of the people that died by the Shadow i.e Sir William Smith, Professor Taylor, etc... who apparently died in a similar fashion as the appearance of the brute were magically turned into Brutes.

The final type of monster is called the Kaernk. It is invisible, but only appears in water and can be "seen" by the splashes it makes. It doesn't appear that often in the game and could be considered the least threatening of the three enemies in Amnesia. It can kill you in around three hits and can be easily distracted by human flesh, which is usually found in the areas they appear in. It should be noted that the Kaernk can only "see" you if you are in the water and will give chase at that point and unlike the Gatherers, the terror music won't play if the kaernk notices you so be cautious. The kaernk doesn't appear in that many areas in the game as it is only in 2 areas in the game where it can harm you, the flooded Archives and the Cistern if you take too long.

There are a total of 29 areas in Amnesia: The Dark descent, here is a way to easily see which areas are safe and which ones are dangerous.
Rainy Hall: beginning area, no dangers are encountered
Old Archives: transition to entrance hall, no dangers are encountered
Entrance Hall: serves as the first main hub, no dangers are encountered
Wine Cellar: contains key items, needs key, danger is here, but only if player intends it
Laboratory: needed to make the acid required to move on, no dangers are encountered
Archives: contains key to Wine Cellar, encounters are only noises/hallucinations
Refinery: serves as the exit from entrance hall, danger is here, but only if player intends it
Cellar Archives: flooded area, first encounter with kaernk, danger is here running is recommended
Archive Tunnels: transition to back hall, dangor is here recommended to CHEEZE IT!!!!!!
Back Hall: serves as second main hub, no danger but a centipede baby fountain
Study: needed for a rod, no danger except gravity/bad parkour
Guest Room:needed for machine room key, danger of not being able to open cabinets and Servant Grunt introduction
Machine Room:serves as elevator fixer, requires key, and 3 rods, no danger except for stupidity
Storage:used as storage for 2 of 3 rods, high danger, consult pg. 1 for help on avoiding death
Elevator: accessway to prison, only danger if you are afraid of elevators in which case, good luck
Prison:very dangerous area split into Northern and Southern blocks consult pg. 1 for help on avoiding the same fate as the one guy in here
Southern Block:first area of the prison, 3 possible encounters, 2 minimum
Northern Block:second area of the prison 8 possible encounters, 5 minimum
Cistern Entrance: serves as third main hub 4 doors
Cistern: used for draining the water to the sewer, only danger if you take your time
Morgue: used to get a vachine from the shrooms in the sewer, dangerous hide upon door destruction
Control Room: used to access cistern and Morgue in the cistern entrance, no dangers are encountered
Sewer: transition from cistern entrance to Nave, VERY dangerous consult pg. 1 for help on avoiding a watery death by blade
Nave:Serves as final main hub, 5 doors, no danger
Cells: taken too once killed after getting all orb pieces, danger, recommended to BOOK IT!!
Choir Entrance: transition to Choir, no dangers are encountered
Choir:torture rooms, 3 orb pieces, very dangerous consult pg. 1 for help on avoiding death
Transept: torture room/Alexander's study, 3 orb pieces, no danger except to your sanity
Chancel:hangs above a bottomless pit, DANGEROUS more orb pieces=more danger consult pg. 1 for help on avoiding a death of bottomlessness
Inner Sanctum: transition to orb chamber from Chancel, little danger
Orb Chamber:end of game, choose you ending (next pg)

In Amnesia, there are 4 different possible endings in the game, 2 of them are considered good, and 2 are considered bad

1. In the orb chamber you must knock over all the pylons (pillars) then Alexander should be killed by the Shadow and you should leave Brennenburg alive. (good)
2. In the orb chamber if you have sucessfully gotten Agrippa's head, when the portal starts to form throw the head inside, you will both die and Agrippa will ask Weiger to bring Daniel with him (good)
3. Let the ritual go on and Alexander will escape and you will die (bad)
4. Back in the cells wait for the Shadow consume the cell and you will die (bad)