I don't know what is behind that door, but I do know that the door often tells me what it is I could find. This isn't necessarily a constant but it shapes the way you perceive what might happen as you pass through the portal to the other side. Consider a door for a moment that you didn't know was there at first. The worlds people often build contains doors like these, secret, for some reason. I will repeat that for clarity. For some reason. Does it make sense that whoever built the structure the adventurers are delving into would just arbitrarily choose a room and say, "just for fun let us make this door harder to find." I'm not doing that this is impossible but maybe it doesn't make sense for every door that is hidden to not have good reason to be. Whatever made this shadowy gateway more likely had no intention of others knowing about it.
My point is that doors have a purpose intended upon their being constructed. Locked, hidden, heavy, metal, wood, doors all have differing makes and most of the time not just to match the aesthetic of the area around them.