Valley of defilement
I'll copy in what I said in another thread:
The greatness of the 5-2 swamp is how, similar to 3-1 but I would argue even more successfully, it makes you feel the same as your avatar would going through the area. It is hopeless, gruelling and absolutely soul crushing. The hopelessness of yourself and your avatar makes you sympathize with the hopelessness of the creatures that lived there, which masterfully ties in with the narrative of the game, as it makes you understand what Astraea must have felt going through here, helping you understand and even sympathize with why she committed her great sin. Is a world which allows such misery to continue even worth preserving? How could a god and powerful god ever tolerate such suffering?
A lot has been said about the poison swamp, but I really want to discuss the second half of the level: the shanty town. And it is great precisely because of the swamp that came before it. You really don't want to repeat the swamp, which makes you into a nervous wreck going through the shanty town. You know that there is a shortcut and that it is within reach, and that if you manage to reach it you will never have to go through the swamp again. But you know that if you die, so close to your goal, going through the swamp is going to hurt so much more, seeing how you almost managed to escape it for good. The game knows that you will feel this way, and uses this knowledge to mess with you. Being so close to the goal may tempt you to rush, but this is the worst thing you can do; the town is full of blind corners where enemies hide. Rush and you will be surrounded in no time. On their own, the enemies are weak, but if they surround you, you can be in a lot of trouble. And the game has another devious trick to make you rush. In its most effective use ever in the series, the poison cloud spell wielded by the shamans keeps you moving, as staying put will get you poisoned. The shamans themselves are always quite a bit ahead of you tempting you to rush them, which would lead you into ambushes.
The relief you feel when you get through the level and unlock the shortcut is immense. For me, it is the platonic ideal of the "I did it" Souls moment. It is probably my most hated level ever, which is what makes it great. The confidence and determination in carrying through their idea for the level to the fullest extent is in my opinion the crowning achievement of the Souls games and what ultimately cements Demon's Souls as the great game of all time