Battle Engine Aquila is a futuristic shooter taking place on a series of war-torn islands on the Earth-like planet of Allium. Two factions, the Forseti and Muspell, battle for control over the world's most limited resource: land. Controlling a monstrous machine called the Aquila, the most powerful weapon in the galaxy, players fight alongside thousands of Forseti troops, aircraft, sea vessels, and tanks as they defend their territory from invading Muspell looking to claim the islands as their own.
Played from a first-person perspective inside the machine's cockpit, players must shoot down all manner of threats, using radar to track enemy units and to assist friendly troops on the 3D battlefields. The insect-like Aquila offers two distinct modes of transport: Walker Mode and Jet Mode. As the name suggests, Walker Mode has players guiding the weapon along the ground using the vessel's four legs and powerful shield generator, which absorbs many types of attacks. Jet Mode diverts power from the shields to allow the Aquila to quickly fly toward its intended destination.
The game is divided into a branching series of 43 missions with optional side objectives for enhanced play. By completing all secondary mission objectives, players can unlock evolved variants of missions designed to offer a greater challenge. Players also receive a letter ranking at the end of each mission. As players make their way across the battlefield, the Aquila will display warning messages such as incoming missiles, incoming warheads, ammunition depleted, and so forth. To help fight these threats, players can outfit their Aquila with pulse cannons, missile launchers, lasers, grenade launchers, torpedoes, spread bombs, rail guns, and more.
Two-player support via split-screen is also featured in three different modes: Skirmish, Versus, and Cooperative. Skirmish has each player taking both sides of the conflict as they try to control specific islands by eliminating as much of the opposing force as possible. Versus is simply a one-on-one battle against two Battle Engines set within a tournament arena. Cooperative unites both players in a quest to defeat the Muspell army, but they can only do so for as long as they both have lives. Additional multiplayer levels are available as unlockable bonuses while playing through the single-player mode.