Door

A Door in Diablo series is generally, like in real life, a moveable obstacle that gives access to another area, often a room or a chamber.

All doors can be opened by clicking on them. Most monsters cannot open doors, but the player's minions can. Doors are often used as bottleneck points to split enemies and fight them one by one.

Game Mechanics
According to the game mechanics, doors are considered as objects like chests and corpses. But unusually, some doors are often immune to Poison.

Some doors can be broken by damage, while others can only be opened manually.

In the first two games, any door that was previously opened (not with force, obviously) can be shut again, but only if no objects, players and monsters are in the doorway (it will be dubbed Blocked if anything is). Corpses can block doors too.

Skills that force movement (such as Steed Charge) cannot carry heroes through closed non-breakable doors.

Special Doors
There is only one special door in the entire Diablo II game world. It is a huge door in the Catacombs Level 4, in Act I after opening which, one can enter Andariel's Chamber.