Heavy Doors are generally guarding something important such as an elevator or a pod access point. Although the doors can usually only be opened by Overwatch Soldiers, Alyx Vance can open them with her EMP Tool, and Barney Calhoun can open some using command panels. Heavy doors can also trigger alarms when opened depending on their importance.
The Combine Heavy Door is, for the most part, a large, movable metal wall. The door, when ordered to open, will divide itself, with the bigger portion moving to the left (when facing it from the outside). The bigger portion has two small, transparent windows. It is usually hard to see out of, due to it being tinted a dark shade and higher than the player's perspective.