From the Super Mario Wiki
Float Castle I is the seventh level of Yoshi's Safari. It is located in the Light Realm of Jewelry Land. It is the first of two floating castles in the game, but the only one that is seen initially on the map. Mario and Yoshi traverse paths above a massive drop. There are many flying enemies like Paragoombas and Koopa Paratroopas, as well as many jumps over endless drops. At the end of the level, Mario and Yoshi leap from the castle to encounter Roy Koopa in a hot air balloon, loaded with many canons. After shooting the cannons and deflating the balloon Morton Koopa falls. Somehow surviving the fall themselves Mario and Yoshi can now move on to the Dark Realm after a short cut scene with King Fret.