In Harvest Valley you’ll fight a tough minor boss fight, which can be made easier by lighting the windmill on fire on the way to the fight. Progress through Huntsman’s Corpse to Harvest Valley, then into Earthen Peak. The path is at the contraption on the way to Heide’s Tower of Flame, but you must exhaust all talking options with the merchant to make her appear there and offer the path. If you exhaust all talking options with the merchant, she’ll unlock the path to Huntsman’s Corpse for 2000 souls. If you’ve cleared out Heide’s you’ll have met the miracle merchant at the second bonfire. Heading towards Heidi’s Tower of Flame from Majula, you may have run into a contraption in the tunnels on the way there. Though you’ll need to get there, because that’s where The Old Iron King resides. Getting to the Iron Keep is a bit more complicated than other areas, and not necessarily explained in the game. You can beat The Old Sinner without using the Bastille Key, it’s just a lot harder as targeting doesn’t work, and the room is pitch black. Getting the Bastille Key will allow you to light the flames on both sides of The Old Sinner’s Boss Room.
They are similar to the Gargoyles from the original Dark Souls. To get the key, you’ll need to beat the Belfry Gargoyles. To make The Old Sinner a little bit easier, you can unlock the doors on the right and left of the mist gate before entering the boss fight, and they’ll give you the opportunity to light the room. The Old Sinner was our first great soul to tackle, and he feels like the most natural character to try first from a difficulty standpoint. You’ll need to fight your way through The Lost Bastille, and fight the Flexile Sentry though. You can also progress through The Lost Bastille by way of the N Man’s Wharf. You don’t have to take the nest route though.
From there, it’s a short trip to Sinner’s Rise, where The Old Sinner resides.
If you beat the Pursuer, the bird’s nest unlocked after that boss fight will allow you to fly to The Lost Bastille. There are a couple of ways that you can get to The Old Sinner.