I finished it when I was at soul level 88, so it was a huge pain for me, even more so because I was a Knight with no spells at the start of NG+, so I was pretty much fucked up, until I switched my strategy from heavy to light armour.
About 4-1. When you reach the top wall above the gate, you'll see on the right side of the wall some stairs leading down. In there you'll find that treasure, it's the key to open the Graverobber Jail, be careful about the trap, it will kill you from behind if you don't dodge it.
Current PC Build
CPU - i7 8700K 3.7 GHz (4.7 GHz turbo) 6 cores OC'd to 5.2 GHz with Watercooling (Hydro Series H110i) | MB - Gigabyte Z370 HD3P ATX | Gigabyte GTX 1080ti Gaming OC BLACK 11G (1657 MHz Boost Core / 11010 MHz Memory) | RAM - Corsair DIMM 32GB DDR4, 2400 MHz | PSU - Corsair CX650M (80+ Bronze) 650W | Audio - Asus Essence STX II 7.1 | Monitor - Samsung U28E590D 4K UHD, Freesync, 1 ms, 60 Hz, 28"
