Sentinels were created by Forerunners as automated worker and defense drones to maintain and protect Forerunner installations from transgressors. Although designed without the independent intelligence of Forerunner Monitors, or the UNSC's smart Al, they remain highly adaptable and a formidable foe to anything they recognize as a threat to the Ark.
The Foundry at the center of the Ark is populated by Retriever and Assembler Sentinels, who extract resources and minerals from the small moon held there. The mining capabilities of the ship-size Retriever Sentinels can be highly destructive when used against trespassers, but it is the smaller Aggressor Sentinels that are primarily used for enforcement.
When deployed in great numbers, these Sentinels will swoop in and attack as one in a perfectly coordinated wave. Swiftly removing any threats while leaving the Ark undamaged.