To obtain Havel's Armor, starting from the first bonfire in The Gutter, turn and jump down to the building below with a ladder. Climb up the ladder, and enter the dark cave that contains poison-spitting creatures. Defeat them, and open the Forgotten Door. Then, smash the vase near the back wall to obtain Havel's Armor, which is of the most useful heavy armor sets from the original Dark Souls.