Since this dungeon not only has 2 entrances but also different enemies/paths depending on whether you’re following Ranni‘s questline or not, we’ll assume that you have gone through the Siofra River dungeon and that Blaidd cleaned up the castle before your arrival. The main difference here will be the castle being mostly empty until you fight Starscourge Radahn. Once he’s defeated, the castle’s garrison will return and you’ll be able to fight an additional boss, which is available straight away if you aren’t following Ranni‘s questline.
The waygate next to the Impassable Greatbridge site of grace.
A ladder in the castle’s cemetery.
Once you enter the dungeon through the ladder found in the cemetery, you might be surprised to see the castle being almost empty. Since Blaidd made his way here before you did, it seems that he’s already gone ahead and killed most of its garrison. What this means for you is that while the castle is mostly safe to explore, several areas will remain inaccessible until you’ve defeated Starscourge Radahn. Additionally, the Misbegotten Warrior + Crucible Knight boss fight will also only become available once you’ve defeated Radahn.
For now, continue heading through the upper level until you reach a ladder allowing you to move to the castle’s lower level. There, after defeating a regular sized dog, enter the nearby room with several weak enemies.
Cut your way through them and unlock the gate leading back to the cemetery that you went through to enter the castle. On top of allowing you to unlock a shortcut, this room also has 2 corpses with 1x Smithing Stone  and Armorer’s Cookbook . With these items now in your possession, go back up the ladder and look for a spot to jump further down into the courtyard. There, head north and grab the site of grace that can be found in a small room to the right: the Chamber Outside the Plaza site of grace.
Instead of going deeper through the castle, come back out and look west to find a staircase leading to the top of a building. There, you will be able to grab a Smithing Stone , and if you jump on top of a tent there you’ll be able to deal with the two lions patrolling the courtyard by using ranged attacks. With the castle being mostly deserted, this will be the perfect opportunity to deal with these lions, as they do not respawn once they’re defeated.
While both of these lions will be able to avoid all of your ranged attacks while looking at you, you’ll be able to land an attack when they lose interest and start heading back to their patrol path. While this is a safe way to defeat them, it may take a bit of time depending on the strength of your ranged attacks.
Every time the lion you want to attack is standing underneath and is dodging all of your attacks, move back a little and crouch out of sight, until they turn around and start heading back. This will create another opportunity for you to use a ranged attack that the lion won’t be able to dodge. If the lions are too far away for you to attack with a spell, don’t hesitate to pull out a bow to get them to come closer, and just attack them once they start heading back. Lastly, if you don’t have access to decent ranged attacks and melee is your only option, make sure to fight them one at a time by using a ranged weapon to grab the attention of one lion at a time.
Defeating those lions will grant you a Somber Smithing Stone  from each of the lions as well as some Old Fangs, and you’ll be able to grab a few more things from the courtyard. Grab the Golden Rune  by going up the stairs inside of the wooden structure nearby, the Smithing Stone  from the pile of corpses near the gate, the Arteria Leaf near the main gate, and head to the area that was past the Chamber Outside the Plaza site of grace.
The area past the site of grace has several NPCs waiting for the festival to begin. Exhaust the dialogue with all of them to get the Polite Bow gesture as well as some bits of story, before going up the stairs and speaking to the NPC on top of the wooden platform to trigger a cutscene, receive the Heartening Cry gesture, and begin the festival.
After the cutscene, grab the Somber Smithing Stone  behind the monument north-northeast and head to the structure northwest that will now have its doors open. Make sure to grab the Smithing Stone  before riding the elevator down to another waygate.
Once you’ve defeated the boss, grabbed the site of grace in the boss room and exhausted the dialogue with the nearby NPCs, travel back to the Chamber Outside the Plaza site of grace in Redmane Castle, and head back to the church that you passed through to get to the Radahn boss fight to find an NPC sitting there.
Speak to Jerren and exhaust his dialogue before heading back to the Chamber Outside the Plaza site of grace and resting twice. Doing so will repopulate the castle, and now the way back towards Radahn‘s boss room will be blocked by a fog gate. Not only do you now have access to an additional boss fight, several doors that were locked inside of Redmane Castle are now open. Since we’re here however, we might as well deal with the Crucible Knight and Misbegotten Warrior boss fight.
With the boss dealt with or skipped (it is optional after all), head back to the courtyard and head south. The door south on top of the stairs is now open, though you will have to cut your way through some enemies. There are several enemies manning stationary flamethrowers, so make sure to deal with them first, to ensure that you do not get hit in the back on your way past them. The room we’re heading to has a ladder that will take you to an upper section that was previously inaccessible. There, a Pumpkin Head enemy is guarding a corpse with a Flamberge and another corpse to the left of it with a Smithing Stone . Additionally, there is another ladder that allows you to climb the nearby tower, which is where you can grab a Smithing Stone .
Now go out of the building and start heading northwest, past the barricade. There, head towards another door that is now unlocked, all while avoiding or fighting the Iron Virgin hiding in a corner to the right.
Fighting it doesn’t yield anything unique, so if you like you can just run inside of the building to avoid fighting the Iron Virgin. Inside, make sure to grab the Red-Hot Whetblade before going up the stairs and going east, where you will find a series of rooftops connected by wooden bridges and defended by several soldiers and a knight. Make sure to grab the Smithing Stone  from the chest in front of the knight, before going back down and heading towards the castle’s main gate.
Having previously defeated the two lions patrolling the courtyard, you can safely move towards the building to the left of the main gate, which is where you will find the last piece of loot left to grab in Redmane Castle: Armorer’s Cookbook .