The Great Hall
From Bandipedia, the Crash Bandicoot wiki
The Great Hall is the second-to-last level in the first Crash Bandicoot game. Despite the exit is accessible as soon as the player enters the level, it has a secret one (a balcony), which is unlocked when all of the gems in the game are gathered. By clearing the hall, you unlock the alternate ending.
Quite noteworthy is how the background music heard in this stage is used three years later, in the last section of the "scrapbook" seen in Crash Team Racing showing the whole Naughty Dog staff.
