A Sentinel Constructor Factory is a massive floating platform that produces Sentinel, Sentinel Major and Enforcer automatons.
Introduction
These facilities float high above the ground, in an attempt to keep them somewhat safe from the majority of Flood if there is ever another major outbreak amid the Installation. On the planet Onyx, there was a Sentinel Production Facility which constructed Onyx Sentinels every six seconds, quickly making a mass of Sentinels prepared to combat any known threat. It was later destroyed by the Spartan-II's and III's.[1]
Appearance
During Halo 2, they first appear in the level "Sacred Icon" when the Arbiter reaches the inner abode of the Sentinel Wall.
There is an opening cleft in the structure that is similar to the opening where he first entered from the Phantom. In the distance, above the Library, there are two large floating objects. Once the Arbiter exits the wall, one of these objects is hit with a few Plasma Mortar shots (possibly fired from a Flood captured Wraith) to slow down the Forerunner defenses which then crashes down to the ground.
In the next level, you would be able to observe the same crashed structure being pounded by Wraiths and Scorpions and fight through it as Sentinels and Flood fight each other[2].
While the appearance inside the structure is badly damaged, some believe it to be a futuristic assembly line allowing the Monitor to have endless waves of Sentinels to control future Flood outbreaks.