There is a red door at the end of Mausoleum and Gehenna 2 in The Binding of Isaac: Repentance, and this guide details how to unlock it.

Clearing Corpse 2 in The Binding of Isaac: Repentance causes a Strange Door to appear in Depths 2, and here is how to unlock it.

Executing the command allows plays to start online co-op sessions via the title’s debug console. This is the first time online co-op has officially appeared in The Binding Of Isaac series.