The Infernal Engine is a large, box-shaped, apparently mechanical Heartless with four black wheels, two on either side of its body. It appears to be predominantly made of wood. Its face takes up its entire front side, with a large, red, jagged mouth with yellow "lips" on its lower half, and a black "forehead" with silver, metal additives on it. Its eyes are a glowing yellow color and its Heartless emblem is in the middle of its face. There is a wooden guardrail on its top decorated by yellow and red patterns as well as four white spikes. The Infernal Engine is able to release a battering ram, hammer, and boxes from its mouth.
Riding atop of the Infernal Engine are several smaller Heartless, referred to as "archers" in the Journal. Each of these Heartless wear silver armor, including a bullet-shaped helmet with an opening for their black faces and glowing yellow eyes. All of their helmets sport two short spikes and a curled, black antenna, and they do not seem to have legs, instead balancing on a spike. They wield large, indigo weapons that fire projectiles and each sports a Heartless emblem emblazoned on their chests.
The Infernal Engine's Japanese name, "Demon's Fortress", reflects its massive, sturdy construct and the way the archers use it as a "fortress" of sorts. This name and the Infernal Engine's behavior are also reminiscent of the Demon Wall enemy from the Final Fantasy series. Its English name has the same root, as infernal can mean "relating to hell", and "Engine" is a reference to its mechanical nature and its use as a vehicle by the smaller Heartless archers. It may be a reference to another Final Fantasy enemy, the Doomtrain.
The battle with the Infernal Engine can be quite difficult. When the three lesser Heartless drive the main boss, they can allow it to perform a variety of attacks. These all appear rapidly, and the flurry of moves can only be slowed by defeating the three lesser creatures. When the smaller Heartless are in control, the Infernal Engine's attacks are as follows:
When the archers have been destroyed, the Infernal Engine will spit out boxes containing Soldier Heartless until they have returned. Just ignore them and keep hitting the Infernal Engine with aerial combos. The Soldiers will eventually disappear. The three archers will respawn in a short time. Utilize the Limit Break, should Roxas hit low HP and remember to heal periodically.
A useful tactic to defeat the Infernal Engine is to eliminate all but one of the archers. Reflect cannonballs using Block until two archers have been defeated, then dodge the rest of the shots. This will ease the process of dodging the archers' shots while avoiding the Engine's diverse arsenal of attacks.
The battering ram is the Infernal Engine's weak point. Capitalize on this weakness whenever the Engine begins to charge the battering ram. Land as many combos as possible, then move away to avoid being trampled.