Onslaught is split into two main modes: Initially, the player selects a territory based on its relative difficulty and influence. Then, as the high-powered "Fanatic" warrior, must fight their way through hordes of enemies in an action-platformer mode to reach the master of the castle. This master must be fought in a "mind battle": He'll be represented as a head with four wavy arms that fires magic at the player, who must dodge fighting back in a Tempest-style shoot-em-up battle. Should the player win, the territory is now theirs and they are able to select the next territory to conquer.
Onslaught is notable for its moody sound track (a hallmark of Amiga games) by Dutch video game music composers Maniacs of Noise. It's also notoriously difficult, like many games of the 8-bit and 16-bit eras.