Found at the very north of Caelid, in Greyoll’s Dragonbarrow, the Divine Tower of Caelid is accessed by climbing the tower’s exterior. On top of allowing you to restore Radahn’s Great Rune, it also leads to another optional boss: the Godskin Apostle.
Unlike the Divine Tower in Limgrave, here you’ll have to do a bit of parkour to be able to restore Radahn’s Great Rune. To start your ascent, go to the cliff next to the tower and jump over to the branch, before jumping over to the small ledge where a soldier is standing guard.
There, ignore the ladder for now and walk on the ledge going southeast to grab a Stonesword Key. As you go around the corner and see the corpse with the loot, watch out for enemy standing behind the corner to ambush you.
Now go back to the ladder and climb it all the way to the top. If you look north, you’ll see another ledge that you can use to cross to the other side, this time however you’ll have to carefully jump over a couple of gaps to safely cross to the other side. Defeat the enemy there and continue by crossing over the next ledge leading northeast, and you’ll reach another ladder going up.
Climb this ladder and you’ll finally be able to enter the tower and get the Divine Tower of Caelid: Center site of grace. While you can start exploring the inside straight away, there is one more piece of lot that we can grab outside of the tower: Numen’s Rune. Go back down the ladder that leads to the site of grace and face east. There, jump down the 2 gaps in the ledges to reach another ladder, which leads to a corpse with the Numen’s Rune.
Now travel back to the Divine Tower of Caelid: Center site of grace and you’ll have 2 choices: a hallway east of the site of grace that leads to the tower’s lower level, and another hallway west of the site grace that leads all the way to the top, which is where you’ll be able to restore Radahn’s Great Rune.
We recommend restoring Radahn’s Great Rune first since it’s a quick and painless thing to do, unlike the path that leads deeper inside of the tower. So just take the path west until you reach an elevator, ride it all the way to the top of the tower and restore the Great Rune to get Radahn’s Great Rune. Now travel back to the site of grace and head east to start exploring the Divine Tower of Caelid.
The stairs lead down to a small room, with a soldier sitting in the back and another one waiting to the right of the doorway. To make it easier, use ranged attacks on the enemy in the back of the room to deal with him before dealing with his buddy. Northwest of the room, jump down to the platform below, and follow the ledge going north along the wall to reach another similar platform that you can jump down to.
This second platform will crumble beneath your feet, which will allow you to reach an even lower part of the tower. There, look for a narrow beam, and drop down by walking forward to the side of it.
Turn around and grab the Golden Rune  before doing some more platforming until you reach an elevator.
Ride the elevator all the way up and instead of continuing down the nearby hallway, climb the ladder and open the door to unlock a shortcut that will allow you to bypass all of this jumping in case of an unforeseen death.
Go back down the ladder once you’ve unlocked the shortcut, and go down the hallway to find a fairly robust enemy. Not only does he have a lot of health, you will not be able to stagger him when he starts casting his incantations. However, don’t hesitate to backstab him as there will be plenty of room to roll towards his back and stab it.
Once he’s dead, go through the hallways where he walked out from and grab the Rune Arc on the corpse at the end of it. From there, walk on the ledge to your left to drop down to another crumbling platform that will lead you to another elevator, which leads to the Divine Tower of Caelid: Basement site of grace.
From this site of grace, go down the nearby stairs, fight another enemy blocking your path, and go through the fog gate to fight Divine Tower of Caelid‘s boss: Godskin Apostle.