The AT-AT Barge is by design a large transport craft. It is capable of transporting and disembarking 2 AT-AT heavy assault walkers, or 6 AT-ST scout walkers. Using maglock equipment it is able to land open it's doors in the base of the transport area and then deploy its walkers combat ready in a matter of moments before lifting out of the combat area. This makes it extremely hard to prevent the barges from landing their embarked combat units.
The transports are heavily armored and shielded. They can survive a large amount of damage before being destroyed in combat. For a transport craft, it is unexpectedly fast and maneuverable as fast as many starfighters. The ship is not equipped with a hyperdrive.
The AT-AT Barge is completely unarmed but its size, heavy shielding, and armor makes this a minor concern. If enemy starfighters are seen as a possible threat, then it can easily be escorted by TIE Fighters.
The Empire deploys these transports onboard all of their Star Destroyers that have troop transport capability. Most Imperial Star Destroyers carry at least 8 of these dropships to allow them to deploy their onboard AT-AT walkers in one deployment action against a planet. As the transports do not have Hyperdrives they are never deployed independently of a larger combat warship.