Combine Smart Barrier

The Combine Smart Barrier, also known as Combine Mobile Wall, is a moving mechanic wall, using powerful drive systems in tandem with heavy metal blocks to slowly expand outward and destroy existing structures in the way. Smart Barriers are seen surrounding the Citadel (where they make up the "City 17 Inner Wall"), certain areas of Nova Prospekt and City 17, notably on the Trainstation Plaza during the Uprising and around the Overwatch Nexus.

Overview
Standard operation is to lift and force a set of blocks outward, before allowing them to drop, and then alternating with another set. During this procedure a metallic groaning is heard, presumably from the drive mechanisms. It is unknown whether the Combine are using this method to simply destroy existing structures, or whether to gather raw materials as well. Either way, they are also used as blockades, blocking off alleyways and even entire streets as necessary. Gordon Freeman narrowly escapes being crushed by Smart Barriers shortly before getting into the Nova Prospekt Depot.

As its name suggests, the Smart Barrier seems to be self-operated and will sometimes get into movement when a being passes nearby, crushing it if he remains in the way.

Behind the scenes

 * As seen in the WC mappack and suggested by the playable Half-Life 2 Beta sound files, the City 17 Inner Wall was originally to be stationary, much higher and made out of concrete. The Manhack Arcade was at some point to be located nearby, before being replaced by Captain Vance's Headquarters, where Gordon was to go look for Vance while Alyx was to be left hurt in the Skyscraper.
 * An early version, only existing as concept art, was made of cubes.
 * An early prototype version, featured in the WC mappack as a brush, was named "Sensor Gate". It can be found in the WC mappack maps " ", " " or " ". It is always accompanied by an early Combine Watchtower (Watchtowers are often seen near Smart Barriers, but also Barricades), often featuring a Metrocop inside. By default in open state, it shuts itself when the player approaches it, also reacting to a nearby presence with its "sensors", much like the final version.

List of appearances

 * Half-Life 2
 * Half-Life 2: Raising the Bar
 * Half-Life 2: Episode One