Belfry Sol can be accessed via a Pharro's device found in the Iron Keep. Beyond the hidden wall is a ladder which leads to a bonfire and then another that leads to the mist door that enters Belfry Sol proper. Next to the mist door is the bellkeeper guardian. You can join the bellkeeper covenant if you talk to him. Invaders will appear frequently in this area (just like Belfry Luna) so come prepared. When you are ready, head through the mist door to black guardian phantoms (small phantoms who use axes, bows at range and can throw small bombs as well). These are exactly like the ones in Belfry Luna. A larger black phantom with a staff will also be with them, so make sure to take the small phantoms out quickly before engaging it,
Travel straight ahead and look for a path to the left to spot another small guardian and a black phantom with a greatshield and large sword. If you are part of the bell keeper covenant another invader may arrive here as you approach by the name of Mad Warrior. He can also drop the Mad Warrior Armour set and a Berserker Blade. Ranged attacks work well against both of these phantoms, melee players will want to hit and run due to the reach of the weapons wielded.
After the black phantom is down, you should have two small towers in front of you. Behind the left one is a body with a Simpleton's Spice on it. The right tower contains a lever that you can pull to ring the bell for the area. Enter the left tower to find a ladder up to the rooftops. A dual crossbow wielding phantom up here will try to pick you off when you climb up. Use ranged attacks or close quickly to defeat him quickly. A nearby corpse will have a Simpleton's Spice to pick up. Further along the rooftop is another greatshield phantom and his two small guardian phantom pals. They will use bows against you (making them much more dangerous) so use a shield or cover when approaching them. The same tactics apply as usual.
The doorway behind them leads to a chest with the Immolation spell inside. Down the stairs next to this is a corpse holding a Human Effigy and Triclops Snake Troches. The doorway nearby leads back to the Iron Keep on a ledge above the bonfire next to the entrance of Belfry Sol. The ledge here has a hidden door containing a pair of chests with a Black Knight Greatsword, Protective Chime and a Grand Spirit Tree Shield. Drop down to enter the Iron Keep area again.
