Like Belfry Luna, this is an optional area in Dark Souls 2, and if connected to the internet, you are likely to be invaded by players from the Bellkeeper Covenant.
As you head up the stairs from the first bonfire (found by using a Pharros’ Lockstone in Iron Keep), you’ll be greeted by a mist doorway and a bellkeeper. Don’t bother joining the covenant, pass through the mist and you’ll face dark Bellkeepers and a dark witch. The Bellkeepers will respawn if you die or rest at a bonfire, but the other dark spirits won’t. Neither the bellkeepers or witch are particularly troublesome.
Once the two Bellkeepers and witch are down, head to the far left of this area, where there’s two rooms. The room on the right contains a lever to ring the bell, beside the other room is a dead body holding a Simpleton’s Spice.
Head into the second room and up the ladder, there’s a dual-wielding archer at the top. He’s easy to kill if you have a decent shield, as he tries to flee when you get close. Once he’s down, a shadow greatsword foe from the other side of the gate will likely come and attack you. He’s also pretty easy to kill thanks to his long recovery rate and slow attacks. In the top right corner of the roof is another Simpleton’s Spice, grab this before dropping down and killing the other two Bellkeepers.
There’s an Immolation spell in the chest in the room ahead, head down the stairs and grab the Human Effigy and Triclops Snake Torches before proceeding through to Iron Keep. As soon as you head into Iron Keep, there’s an illusory wall to the right leading to two chests (before you drop back down to the bonfire where you started), inside the two chests are: the Black Knight Greatsword, a Protective Chime and the Grand Spirit Tree Shield. Head down the ledge and back to the bonfire.
Dark Souls 2 Guide - Belfry Sol walkthrough