In the Gates of Hell, it takes 3 Scatters (skulls) to get to the Hellfire Spins, which are akin to free spins. The main goal of the bonus game is to beat the boss characters, however, the game grid also increases to 6x7. Here are the basic rules:
You start with 5 health points.
Boss characters persist through spins.
Defeat the boss to gain more health points.
Bosses always appear in the same order: Baron Twins, Infernal Spider, and then Hellfire Tyrant.