Forest of Fallen Giants
Proceed through the area near the Cardinal Tower Bonfire until you reach Pate. Make a right near him to spot an archway with some ballistas. Step inside, then back out quickly. The undead soldiers will fire the ballistas, then pursue you. Take them out, then head inside. On the lower level is the Pharros Contraption; use your lockstone to reveal a secret room hidden by a fake blue wall. Inside is a pair of chests with a Titanite Slab and a Chloranthy Ring.
There are several sets of stairs embedded in a hill in this area. When you ascend the first set, make a right. Take out the bandit and his dog, then make another right to find this contraption on the ground. This won’t net you any treasure, but it will illuminate the entire cavern, making navigation easier and scaring the poisonous enemies indoors.
The Lost Bastille (1 of 2)
This is found directly beneath the Servants' Quarters bonfire. Climb down the ladder in the corner to find it, then use a lockstone to reveal a fake blue wall. Head through to gain access to a new area, Belfry Luna.
The Lost Bastile (2 of 2)
Found in The Last Bastille. When proceeding from the Servant's Quarters bonfire, you'll have to jump across some rooftops. Drop down, take out the dogs, and head down this alleyway. Climb the ladder, hop through the window, and take out the mini knight on the other side. Continue into the next room, and check the wall to your right to find another contraption. Use a lockstone to reveal two fake blue walls. Behind one is a Soul Vessel; behind the other is a Wilted Dust Herb.
Doors of Pharros
Here's the deal with the Doors of Pharros - every Pharros' Contraption here creates traps meant for the Rat King covenant. There are minor prizes behind some, but by and large, you should not waste your lockstones here.
Earthen Peak (1 of 2)
Upon entering Earthen Peak, you'll be in a chamber with poison water below. Step down the stairs, then follow the hall to the right, then take the first right. Take out the poison enemies here, then look along the wall for the contraption. Using a lockstone reveals a fake blue wall. Behind it is a chest containing the Poisonbite Ring.
Earthen Peak (2 of 2)
Climb Earthen Peak until you reach the poison pool before the fog gate. To the right of the gate is a lit passage; follow it, then make another right. In here are several poisonous enemies and pots, plus a contraption. Use a lockstone to create a pool of poison; once you set the windmill aflame, this pool becomes water, which heals poison and restores health.
Iron Keep (1 of 3)
You can find this after lowering the first bridge in Iron Keep. Cross over, then make a right. From here, you can leap to a lower ledge near the lava. Do so, then enter the small room and climb the ladder. Take out the knight here, then use a lockstone on the contraption against the wall. This creates a pool of water. Stand in this, and your character will get wet, giving you some resistance against the fire and lava in the area.