Donkey Kong Country 2: Diddy's Kong Quest · Super Nintendo (SNES) · 1995
A hidden world buried beneath Crocodile Isle, gated behind a Kremling toll-keeper who demands 75 collectible Kremkoins earned only by clearing the game's hidden bonus rooms.
Rare packed Donkey Kong Country 2 with hidden content, but its deepest secret is the Lost World — a separate set of five stages that house Krocodile Kore, the power source of Crocodile Isle. Access is sold by Klubba, a hulking Kremling who mans a tollbooth (Klubba's Kiosk) in every world and charges 15 Kremkoins to open each Lost World stage, 75 in total. Kremkoins are earned by completing the hidden Bonus Rooms scattered throughout the game, so reaching the full Lost World demands near-total exploration. (The Banana Bird caves often associated with this idea actually belong to the sequel, Donkey Kong Country 3.)
Complete the hidden Bonus Rooms in each level to collect Kremkoins, then visit Klubba's Kiosk (present in every world) and pay 15 Kremkoins to open each of the five Lost World stages — 75 in total.