| Doors
| Doors are barriers that can open or close, allowing or preventing the passage of characters and objects. Some doors can be opened or closed by the player or by characters, while others may be controlled by routines. For example, while the player might be unable to open a particular door through the usual method of approaching it and pressing the action key, he might instead be able to turn on a switch that would perform a door opening routine. Doors may also be configured to open themselves automatically when a character is nearby, or after being sufficiently damaged by projectiles. The appearance of a door and the sounds that it makes when it opens and closes depend on the door's door type. A door type includes four animation sequences: Door Open, Door Closed, Door Opening, and Door Closing. Each animation sequence may consist of any number of frames. Doors are simple objects with no true physical properties. They can not move or disappear. When a door closes, it effectively turns the area that it occupies into solid material. Another way to create solid material is through a routine that inserts a block of material into a level. Doors, however, have the added benefit of being able to change their appearance to indicate their solidity. Information on editing doors is available in the Door Creation section. Doors may be added to levels and manipulated as described in the Adding Level Objects and Level Object Manipulation sections.