It’s easy to underestimate the humble door. You open it, you go through. Sometimes, you must find the key first, and for many games, that’s the whole extent of the player’s interactions with doors. They’re something to get past, something that cordons off one bit from the next bit. A simple structural element, of special interest to level designers, but not the ones who turn the knobs.
And yet, the fundamental nature of doors that makes them seem so mundane also imbues them with a kind of magic. How do I open it? And what could be behind it? A good door is a locus of challenge and mystery; mystery that could give way to delight, wonder, or even a good scare. A good door is a teasing paradox that does everything in its power to entice and invite, but also puts up a decent effort to keep you out, at least long enough to intrigue and fire up your imagination.
Some games highlight the versatility of doors by turning them into especially dense knots in the possibility space. In games like Thief, Dishonored, Prey, Deus Ex or Darkwood, doors can be lockpicked, hacked, blown to bits or cleverly circumvented. In emergencies, they can be barricaded, blocked by heavy objects or even taken off the electric grid. For the tactically minded, they can serve as choke points to lure enemies into traps or ambushes, while the patient can use keyholes to spy on the unsuspecting, or simply get close enough to a door to eavesdrop on an important conversation.