The RUINS are full of puzzles. Ancient fusions between diversions and doorkeys. One must solve them to move from room to room. Please adjust yourself to the sight of them.
Puzzles are a prominent component of Undertale, and are explained to the protagonist by Toriel. Toriel assists the protagonist in solving the first three puzzles in the Ruins, then promptly abandons them.
The Middle Road Puzzle
The middle road puzzle is the first puzzle that the protagonist encounters in the Ruins. This puzzle features six switches placed on the ground and a lever. A plaque on the wall reads "Only the fearless may proceed. Brave ones, foolish ones. Both not walk the middle road." This gives the protagonist the hint that they need not step on the switches on the marked path. Solving this puzzle opens up the door to the switch puzzle.
However, Toriel solves this puzzle in front of the protagonist before they can interact with the puzzle.
The Switch Room
The switch room is a room with three switches on the wall, two bridges, spikes that prevent the protagonist from entering the next room, and a plaque on the wall that reads "Stay on the path." This quote gives the protagonist the hint that they need to hit the switches that can be reached on the path.
The labels "Please press this switch" and "Please press this switch too" have been added by Toriel next to the switches that the protagonist needs to press. Toriel stops the protagonist if they attempt to flip the wrong switch. In Hard Mode, the labels have faded away, making the puzzle marginally more challenging.
If the protagonist manages to pull the third lever while Toriel is still walking, the message "Wow! You are super fast at being wrong." is displayed, as the "Would you smooch a ghost?" correct answer sound plays.
The Blueprint Puzzle
The blueprint puzzle is a two-part room where one side of the room features markings on the floor, and the other side features spikes and water. The plaque in the middle of this room reads "The western room is the eastern room's blueprint." This hint means that the markings on the floor resemble the way to get across the spikes on the room to the right. However, the protagonist cannot actually walk on the real spikes since they act as walls.
When coming to the spiked portion of this puzzle for the first time, Toriel holds the protagonist's hand and walks with them across the spikes in the intended manner.
The Rock Puzzle
The rock puzzle consists of two rooms, each with a barrier of impassable spikes and rocks waiting to be pushed onto switches. The plaque in the first room reads "Three out of four gray rocks recommend you push them."
The first room can be solved by pushing the only rock onto the white tile, which deactivates the spikes. This is the first puzzle that the protagonist solves alone.
The second room contains the other three rocks and white tiles those rocks can be pushed onto. This can be solved by repeatedly asking the bottom rock to move to the switch. Once the protagonist approaches the deactivated spikes, the rock moves again, which reactivates the spikes. From there, the protagonist needs to tell the last rock to stay in place. This is a reference to the plaque earlier in the Ruins. The two rocks above this reluctant rock do not need to be pushed to proceed across the bridge.
The Leaf Puzzle
Between the two rock puzzle rooms is a two story room with numerous cracked tiles on the upper floor, some allowing the protagonist to fall through. If the protagonist falls, they are dropped onto the leaves found on the bottom floor.
The plaque on the bottom floor reads "Please don't step on the leaves." indicating the arrangement of the leaves on the ground found on the lower floor is the solution on how to walk across the cracked tiles on the upper floor. Reading the plaque on the upper floor shows the message "Didn't you read the sign downstairs?" since the protagonist has to be stepping on the leaves to read this plaque.
If the protagonist falls ten times, the Legendary Hero consumption noise (fanfare) sounds, and they can freely walk through and complete the puzzle. However, they can also glitch through the wall above the exit and walk right off of the screen, in which the only path open to the player continues only to the right. After walking to the right for around 5 minutes, the walls' hitboxes stop, allowing the protagonist to free-roam outside of the room.
The One Switch Puzzle
The one switch puzzle is a room that has six cracks on the upper floor. Each of these cracks leads to smaller individual rooms with leaf piles in them on the bottom floor. The plaque on the wall reads "There is just one switch." hinting that the protagonist needs to trigger one switch to solve the puzzle in this room.
- The top left room contains a Vegetoid.
- The top middle room contains the switch needed to progress.
- The top right room contains nothing but a leaf pile.
- The bottom left room contains the Faded Ribbon.
- The bottom middle room contains Napstablook, who fades away after being spoken to. If the protagonist was aggressive towards Napstablook, they do not appear in this room.
- The bottom right room contains another Vegetoid.
Solving this puzzle disables the spikes and allows the protagonist to progress into the rotating puzzle room.
The Rotating Puzzle Room
The rotating puzzle room is the last puzzle found in the Ruins. The puzzle consists of the same room rotated four times and a room that brings the protagonist back into the first room if the protagonist hits the wrong switch.
- The first room has a plaque that reads "The far door is not an exit. It simply marks a rotation in perspective." The protagonist does not need to do anything in this room since the spikes are deactivated.
- The second room has a plaque that reads "If you can read this, press the blue switch." If the protagonist fails to solve the puzzle in this room, the flavor text points to the blue switch behind the pillar near the entrance.
- The third room has a plaque that reads "If you can read this, press the red switch," which can be found in plain sight.
- The fourth room has a plaque that reads "If you can read this, press the green switch," which can be found behind the pillar in the top-left corner of this room.
If the protagonist repeatedly fails to solve the colored switch rotation puzzle by not finding the first blue switch behind the pillar, the message "You notice a switch hidden behind the pillar" appears. This can be triggered by interacting with the wrong switches and walking back and forth between the second "Colored Switch Rotation" puzzle room.
- "can you believe this puzzle used to be harder. jesus christ. also there were no markings or cracks on the ground" - Toby Fox. September 24, 2015. Twitter.