While "Sonic" games often have unique game mechanics and stories, they feature recurring elements such as the ring-based health system, level locales, and fast-paced gameplay. Games typically feature Sonic setting out to stop Eggman's schemes for world domination, and the player navigates levels that include springs, slopes, bottomless pits, and vertical loops. While Sonic and Eggman were the only characters introduced in the first game, the series would go on to have a large cast of characters; some, such as Miles "Tails" Prower, Knuckles the Echidna, and Shadow the Hedgehog, have starred in spin-offs. The franchise has crossed over with other video game franchises in games such as "Mario & Sonic", "Sega All-Stars", and "Super Smash Bros."