In Dark Souls, there are hidden areas that are masked by Illusory Walls - walls and debris that appears real until it is struck - either by an attack, rolling into it, or kicking.
There is 1 Illusory Wall in Darkroot Garden.
When you reach the door that has been sealed by the Crest of Artorias, look to the left of the path leading to the Forest Protectors to find an Illusory Wall with a Bonfire behind it.There are 3 Illusory Walls in Blighttown. At the base of the swamp, head to the far corner of the poison swamp to find a large treebranch with an Illusory Wall that leads to a treasure chest. The second Illusory Wall is located just behind the chest, and opens up a secret path to The Great Hollow. The third Illusory Wall is found in Queelag's Domain at the bottom of Blighttown. From the stairs by the second Bell of Awakening, you can find an Illusory Wall nearby leading to both the Chaos Servant Covenenant, as well as a hidden Bonfire. Note that by using Seek Guidance, you can find a Developer message on the ground pointing you to the Illusory Wall. There is 1 Illusory Wall located in Sen's Fortress. Head to the tall ladder that leads out of the tar pit, and you'll find an Illusory Wall at the top of the ladder that creates a path to the rooftop above the gate. This leads to a fight with the Giant that opened Sen's Fortress. Note that this particular wall must be attacked - and won't disappear if rolled into. There are 2 Illusory Walls located in Anor Londo. Once you reach the bonfire in the same building that the Silver Knights are located in, look on the same floor as the bonfire to find a fireplace that doubles as an Illusory Wall. This leads to a room with treasure chests that contain Havel's Armor - and a Mimic! The second Illusory Wall can be found from the rotating bridge - head down to the bottom floor and equip the Darkmoon Seance Ring by the Darkmoon Tomb Bonfire, and the Illusory Wall will be removed to reveal the location of the hidden optional boss, Dark Sun Gwyndolin, and the Blade of the Darkmoon Covenant. There are in fact several Illusory Walls in close proximity found in the Painted World of Ariamis. Heading to the underground area by climbing down the well, there's a series of narrow caves and Skeleton Wheels. Many of the flat paneled walls here are illusory, and look different than the other walls. There is 1 Illusory Wall in The Catacombs. Look for a ladder near the area with the lever that controls the second rotating bridge, and you'll find an Illusory Wall flanked by statues that shoot spike traps. Behind the wall you'll find a hidden Bonfire. There is 1 Illusory Wall located in the Tomb of the Giants. After the first bonfire, head towards the tomb where you find the Large Divine Ember, and you'll find an Illusory Wall along the way before the tomb - it's found on the left wall, and leads to the exit from the pit you can get kicked in thanks to Patches. There is 1 Illusory Wall to be found in the New Londo Ruins. Scout the place between the water-logged area and the Ghost-filled building, and you'll find a Darkwraith enemy outside. He stands near an Illusory Wall that leads to another Darkwraith and a chest with a Titanite Chunk. There is 1 Illusory Wall you can find in Lost Izalith. When you reach the second large structure in the lava-filled area, you can find an Illusory Wall here that leads to a hidden Bonfire. The following places are only found in the DLC areas. There are 2 Illusory Walls that can be found in the Oolacile Township. The first Illusory Wall is found near two-story building that houses many Oolacile Sorcerers and the Dark Orb spell - look for a walkway connected to the building and a nearby Developer message that hints at using Cast Light or another light source to reveal the fake wall. The second Illusory Wall is found in the upper floor of the building full of Bloathead enemies. Look for a wall near the broken floor that you would need to jump across to reach a treasure chest, and use Cast Light to reveal a new path to the chest instead of jumping. There is 1 Illusory Wall (and floor) that you can find in the Chasm of the Abyss. Upon reaching the sloping path by the first group of Humanity Phantoms that you encounter, take a left at the next fork in the road - you'll reach a pit as the floor vanishes, and spot an illusion of what appears to be Alvina. The wall this illusion vanishes behind is an Illusory Wall leading to her friend.