The door frame is covered with Spikes
Going through the door (in either direction, in or out) will cause half a heart of damage to the player (and even one full heart of damage later on)
The door leads to a Curse Room
- Do not go through the spikey door if you have only half a heart less, because you will die
- If you have a one heart left, when you go through the door you will take half-heart damage and survive, however you will die when you leave, unless the room contains an item that gives you health or otherwise helps you. So usually it is very risky.
- Otherwise, if you have more than one heart of life, you should definitely enter the Curse Room, as in many cases it contains Red Chests or other powerful rewards.