Unlike the Ruins in Undertale, the ruins in Underfell are quite harsh and are filled with deadly traps and powerful enemies.
Unlike the Ruins in Undertale, the Ruins in Underfell are completely red. The floor appears to be a dark shade of orange, with several cracks running down the walls. Several blood stains are found in the ruins at random locations, as the plates giving the player hints to solve the puzzles have been destroyed by Toriel. Instead, Flowey guides the player safely through the ruins. The spike set puzzle mid-way through also appear to be stained with blood. The dummy, appearing mid-way, appears beaten and torn.
The player must complete each puzzle by themselves. The first puzzle at the beginning of the Ruins will impale the player if the wrong button is pressed. The dummy will be encountered mid-way through. Deadly enemies are found more commonly throughout the Ruins. The spike puzzle will kill the player if the wrong spike is pressed on. The player meets Toriel at the end of the long hallway. The switch puzzle encountered before the end of the Ruins and will cause the player to plunge into a hole to their death if the wrong switch is pressed. Toriel then attempts to lure the player into her house.
Instead of the rather calm theme for the Ruins in Undertale, the Underfell theme emits a feeling of dread and terror.
Instead of a perfectly normal dummy, the one in these Ruins appears distressed, as several tear marks and holes line his body. The eye appears to have been ripped out, dangling by just a thread. The player has the option to spare the dummy.