As opposed to most other special rooms, a player can't tell this type of room from the appearance of the door without items that reveal this information, like The Compass or Spelunker Hat, making the encounter a surprise. Once entered, all doors will be barred with two horizontal wood planks and metal, making it impossible to escape by blasting them open with bombs.
It has three properties that are shared with many special rooms:
- Not every floor contains a mini-boss room
- No floor may contain more than one
- It is always a dead end, except for a fight against Greed or Super Greed in the Secret Room.
Standard Mini-Boss Rooms can spawn on any floor up to Chapter 5. In Chapter 6, Mini-Boss Rooms do not appear except for Greed or Super Greed in the Secret Room. However, mini-bosses are frequently encountered as regular enemies.