The Belfry Sol is a hidden area (just like the previous Belfry level); so let’s point out first how to access it. You need to head to Iron Keep and then proceed until you reach large room with the Alonne Knights , the Ironclad Soldiers and a lever that drops the middle platform into the lava.
To your left is a door and a Pharros Lockstone face to your immediate left. Use a stone on it and it’ll reveal another hidden wall beside you; where you’ll now need to climb the long ladder up until you’re on the upper level.
From here you should look behind you for a metal chest containing a [Thunder Quartz Ring +1] and then you can drop down the small ledge near where you came out for a Bonfire that’s linked to the Sol arena.
Like before with the Luna arena; as soon as you’re ‘in’ this area you’re open to invasions by up to two ‘Bell Bros’ (even if YOU are a ‘Bell Bro’ yourself). There’s the Dark Dwarves up here along with a couple of NPC-controlled invaders to deal with.
Your goal (if you don’t want to fight anyone) is to ring the bell in the nearby tower (to unlock the fog gate exit) and then make an immediate bee-line for it. An invaders job is to stop you from reaching that fog gate.
Once you pass the fog gate you can open the metal chest here for the magic spell [Immolation] . At the base of the stairs (out of view) there’s a corpse holding a [Human Effigy] and [Triclops Snake Troches] .
And that’s pretty much it for this area. If you run down the nearby tunnel you’ll reappear in Iron Keep and thus no longer in the Belfry zone.