A lonely fortress set upon the blasted cliffs of the Mountaintops of the Giants, Castle Sol is an optional setting in Elden Ring that is nevertheless quite important. The castle itself isn't that amazing, but what's in it will allow you to eventually access one of the most difficult bosses in FromSoftware's gaming library.
This guide will help you get through Castle Sol in Elden Ring. First, though, you need to find it, and that requires a lot of exploration.
Finding Castle Sol
Castle Sol is located on the Mountaintops of the Giants, one of the game's final areas, and the last major region you'll unlock (unless you count the next dungeon as a region - it's certainly large enough to qualify). To reach the Mountaintops you must make your way through Leyndell, Royal Capital, defeat Morgott the Omen King, speak to Melina at the nearest Site of Grace, receive the Rold Medallion, and go up the Grand Lift of Rold. Phew.
Once you're on the Mountaintops of the Giants, make your way north through the area until you find the Freezing Lake Site of Grace. Normally you would head south to proceed further through the area, but this time you want to get up onto the cliffs to the left of the Site of Grace and head west. Look over the edge of the cliffs on your right as you ride and you'll be right beside the castle. Assuming you don't want to just leap onto the parapets you'll need to follow the cliffs west, descend to ground level, and ride back east. There's a Death Rite Bird in the fields before the castle, but you can easily run right past it.
Activate the Castle Sol Main Gate upon arrival. Deal with the Walking Mausoleum if you wish, then head around the north side of the castle. There's a graveyard at the rear of the castle where you can find a Golden Rune , a Golden Rune , and a Golden Rune . Ride all the way around the perimeter and you can find a Somber Smithing Stone  on a ledge, just before you return to the Site of Grace.
Castle Sol Main Gate
Immediately inside the main gate you'll see a pair of large, beastly wolves with blades on their legs. These things are quite tough, and fighting both of them at the same time is danged near suicidal. If you insist on fighting, summon a strong Spirit Ash to help you out - or draw one of them to the gate and fight from outside the castle. Otherwise, sneak in if you can or just run for higher ground.
(Assuming you get to explore the lowest level of the castle, you can find a Golden Rune  to the right of the front gate, and there's a Smithing Stone  under the scaffold in the east.)
From here you can advance two different ways. We'll start by going up onto the scaffolding at the other end of the castle from the entrance. This leads to an upper level where you'll be attacked by wolves of a more reasonable size. Check the body on the left for three Thawfrost Boluses, then walk north and into the castle.
Loop through the next room and up a level. You'll be back outside. There's a set of stairs on your left leading to a Golden Rune  and more straight ahead. As soon as you start walking a group of phantasms will appear, all soldiers from Stormveil Castle - albeit with teleportation skills. Kill the one approaching you, avoid any incoming bolts, take out the knight on the stairs, then kill the archer.
Check the ramparts the one archer was firing from for Thawfrost Boluses, then enter the church in the south. There's an Eclipse Shotel in the main shrine, and to the left is the Church of the Eclipse Site of Grace.
We'll start by taking a left at the entrance and weaving through the spiked barricades. Go up the stairs and head east. There's a room ahead and on your left that's filled with slugs. Kill them to retrieve the Nascent Butterfly, then climb the ladder to the left of the body the item was on. You'll emerge on the ramparts, near five Fan Daggers.
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Church of the Eclipse
Take the door on the left side of the church back out. If you go down the path to the right you'll find a ladder and a drop down to a Rune Arc. Hop down to the Rune Arc and you'll find a quick walk to a room that's filled with rats. Kill the rats, check the corner of the room for a Smithing Stone , and go up the nearby ladder.
You're back out on the ramparts. To the left is a gap in the wall that you can leap through to reach a lower section of the wall. Standing out here is a knight with two swords who can demolish you if you're not careful. Ranged fighting is advised if you can; if you're weak in ranged combat, however, at least use something to ping him so he'll walk over. You can then jump attack him. The knight is guarding a Cerulean Amber Medallion +1.
A ladder along the ramparts will take you around a tower, up behind a guard, and then back to the side of the church. Refresh yourself, then head into the doorway across from the church. There are stairs on the left, but if you try to go up three hunched creatures will emerge from hiding spots on the lower level and try to grapple you. This hurts pretty badly, so draw them out one-at-a-time with ranged attacks and kill them. There's a Smithing Stone  on the second level.
Before you go up to get the Smithing Stone, however, look up. There's a stunted man in a cage above this area, and he will scream if you go up the stairs. This will summon two soldiers to block your path. A second screamer further up will summon a two-sworded knight, further complicating matters, and with another knight outside, in front of the boss fog... well, you've got your hands full. Kill the two screamers with ranged weapons before you proceed up the stairs.
And, yes, the boss fog is at the top. Not ready for the boss yet? Good, because there's more to see in Castle Sol. Near the boss fog is a lift that you can activate with a lever. Descend on the lift and you'll find yourself near a Furlcalling Finger Remedy, as well as a pathway to a room filled with slugs. (You could have come here earlier, but we're taking a roundabout route.) Grab the Nascent Butterfly from the body by the ladder, then go up the ladder.
You're back on the ramparts. Grab the nearby five Fan Daggers, then get on the wooden pathway around the closest tower. There's a soldier on the other side, and you'll start getting bombarded with Fire Pots. Kill the soldier, then run onto the next section of wall - and start weaving, as another soldier on the other end of the wall has a ballista, and he'll start firing it at you. The sooner you get over and kill him, the better. Double back to the other side once he's dead to grab a Golden Rune  if you didn't snag it before.
There's a rickety bridge here that's guarded by the Fire Pot flinger and another guy on a ballista. Snipe them with a ranged weapon if you can. Otherwise, wait until both of them have taken a shot, then rush across the bridge and take them out. If there's too much incoming fire you can also hide on the ramparts, bringing the Fire Pot guy out to look for you.
Once these two defenders are gone you'll find a ladder into the castle's central building. It leads to the "Sorcerer" Panting. Grab it and pop open the door on the right. A wolf is right outside the door on your left, so kill it, then take out the other two that you fought before. A hawk will descend into the middle of the fray, as well, so be ready to fight.
That's all for Castle Sol, aside from the boss. You can get back to the elevator leading to the boss fog by hopping down from the small wooden bridge near the wolves.
Ah, joy, another military leader. The head of Castle Sol, Commander Niall should look familiar to anyone who braved the Swamp of Aeonia, though this iteration is worse than Commander O'Neil, primarily because his summons are a pair of knights. One has a sword and shield, one has two swords. Great. Some tips for beating him:
- Take out Niall's knights first. Once you do Niall will pick up the pace and become much tougher to beat - but he'll also do this while the knights are still alive, if you inflict enough damage to him. Fighting three capable opponents rather than two and one laggard is a bad idea.
- Equip items that will slow down Frostbite buildup. Most of Niall's halberd swipes and whirlwind moves will cause you to quickly accumulate Frostbite, and abruptly losing a massive chunk of health can end the fight really quickly.
- Keep away from Niall as much as you can. This is easy enough for ranged fighters, but a problem for melee fighters. If you're taking Niall on with a close-range weapon then you'll want to run in, jump attack him once or twice, and retreat. Keep away from him when you're not attacking or you'll likely get caught by the ridiculous range of his whirling snow attacks.
- Watch Niall's leg. If he lifts his leg he's either going to stomp down and generate a small wind vortex, or, much worse, channel lightning into the leg and try to slice it with you. Both moves are painful, but the lightning leg can absolutely murder you.
- Bring in a summon. Seriously. Fighting the two knights alone is enough to get you killed. Add in Niall after that, and... you're in for a rough time. Make sure your summon is tough or Niall's whirling attacks will kill them quickly.
You'll earn 90,000 Runes and the Veteran's Prosthesis for defeating Commander Niall and his knights. Behind Niall is a tower, and if you go up the elevator in the tower you'll find the Haligtree Secret Medallion (Left), which, when paired with the other half, will allow you to access the big, mysterious spot on the west side of the map of the Mountaintops of the Giants.