Super Mario World/Valley of Bowser 4
From StrategyWiki, the video game walkthrough and strategy guide wiki
To get to secret exit, get Yoshi in the beginning. Keep him till you see the key near the end. Use him to grab and hold the key in his mouth, then walk into the keyhole.--This leads to Star Road, the real exit is not to far, just 2 jumps and you're done.
