Cellar

The Cellar is one of the environments in Chapter 1. It is unlocked after defeating each boss in, including defeating the smallest twin first in Gemini's fight. It is not necessary to defeat Steven to unlock the Cellar. The theme of the Cellar is spiders and poop.

The Cellar's alternate floors are and the.

After is unlocked, a locked door similar to the one leading to a Treasure Room will appear in this floor's Boss Room upon defeating the boss leading to  /. Like a Treasure Room, this door requires a key to be unlocked. Unlike Angel Rooms and Devil Rooms, this door never disappears.

Monsters

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Trivia
In the original Binding of Isaac, Gurdy Jr. could appear as a boss.

Music
The music for the cellar is titled Periculum. Periculum is Latin for "danger" or "risk", signifying how the Cellar is a step-up from the Basement.

Binding of Isaac: Rebirth OST