I say: what the meadow & tree was to the moon, the twinmold desert was to the sun. In other words, you fight twinmold in the sun.

-I also say the Fierce Deity and Majora were enemies that always fought. Eventually, maybe even as far back as creation, they were sealed as masks in the moon and sun respectively
-The stone tower people found the mask in the sun in a manner similar to what Hylian Dan has stated and used it in 'Hexing Rituals'. Meanwhile the mask grows in power.
-Some disaster, probably Majora related, caused the fall of both Ikana and the Stone Tower people and the Mask was spirited away from Termina
-The the mask salesman finds it
-Majora seeks to destroy Termina, as is in it's nature to do. And, killing two birds with one stone, summons the moon to earth to reunite with it's nemesis the Fierce Deity. Meanwhile the mask grows stonger.
-The mask reaches the moon, finds it's nemesis still a mask and gives it to Link so that he can wear it and they can fight.
-The combined strenght of Link and the Deity is too much for it and Majora is destroyed.

P.S. the tower didn't flip upside down. Gravity itself was merely reversed.

P.P.S. This is just fun speculation