Knowledge Engineering: From your 'extensive' knowledge of doors you know that they can be made of various materials and work by bring attached to a frame of some sort with a hinge. They swing in or out sometimes even both. In this case the hinges look like they open outwards. Also there is a hole in the door. Your knowledge of doors tells you it's a key hole. A piece of metal with a unique shape is placed in it to unlock the door called a key.<br/><br/>Knowledge Arcana: Magic and doors go together like owls and bears. It's a lot of work but it keeps people out of places they shouldn't be. There can be lots of ways magic can be used to keep out undesirables from magical traps, locks and just magic to stop magic. These all can be used to stop people from getting through your door.