1. Are there doors in your game? Let's say for the moment there are. OK

2. Can the player open them? Yes. If you have doors in a 3D game and they don't behave like doors, you have failed. Not every door can be opened in RL. Why should it be possible in a game?

3. Can the player open every door in the game? Yes. See point 2. Then you have to provide every possible room for a location even if it is not necessary for the game. Why should I be able to open every door of every house in a street only because there is one house I have to visit?

4. What tells a player a door is locked and will open, as opposed to a door that they will never open? It's a door. It opens. Again, you have a street with houses. Why should it be possible for the player to enter each house on the street? You cannot do that in real life, can you?

5. What happens if there are two players? Doors behave the same for all players. It's a door. See point 2. No, because one player could be allowed to open the door as owner of the house, apartment. The second one is not allowed to open that door.

6. Does it only lock after both players pass through the door? See point 5. See, my reply to point 5. Who locks the door? Is it possible that the invited player enters the apartment and lock the owner out of his own place?

7. What if the level is REALLY BIG and can't all exist at the same time? Then your technology is not good enough to implement your vision and one or the other needs to change. See point 2. This question is about the technology. What technology to chose to make doors in the huge world work. But because doors normally have only a local impact on the environment this is really not that difficult I would say.

I never had the idea to look for C64 based nudity ... until now. And I can't stop looking for all the "games" I had in the 80s.

And why this post currently rated informative?