Underneath certain rocks, or if you bomb out the carpet from the Bedroom, you will find a trapdoor. If it leads to the Retro Treasure Room, the trapdoor will have a visible ladder in it. It can also appear when using We Need To Go Deeper!
The Retro Treasure Room is a small area which, unlike the rest of the game, is in the style of a platformer. There are two ladders, the second one leading to an item from the treasure room pool. Make sure this is the RTR trapdoor if you have not yet completed the floor. RTR trapdoors have a ladder in them, while regular ones do not. Both can appear in the bedroom.
There is a chance that instead of treasure, the RTR will lead to a Black Market, instead.
The Retro Treasure Room is based on the treasure room from The Legend of Zelda.
If you leave and re-enter when there is still at least 1 enemy in the room, additional treasure will spawn.