The "Secret Base" is the final level of Balloon Kid and Balloon Fight GB. On the map of Balloon Fight GB, it appears to be a large building inside an opening of a mountain, and is only accessible by a connected Cave.
The stage starts out with a few Oil Drums, and then has a section where you must drop your Balloons and walk along the ground. You then face some Moving Platforms, some Crushers, and the ground is covered in lava. You then must re-inflate your Balloons to fly across a portion with many obstacles and lava. Soon afterward, you arrive at the boss room, where you must defeat the Robot to save Jim.
- Since this is the final level and has the final boss, beating this level will initiate the ending cutscene and then the credits.
- In the large portion of the level with lava, there appears to be Ultra Hands sticking out of the lava. The Ultra Hand was a toy which was made by Nintendo, before they moved into making video games.