The Tower of Mastery is a building in Shalour City. It is said to be where Mega Evolution was first mastered in Kalos. It had a prominent role in a series of RPs between Paigelena and HeroBlast before the latter had joined the Pokecrew.
It has appeared once in a major role where Gothi was trying to prove to Vanessa that she was evil enough to be her apprentice. She proved this by hypnotizing Korrina to be her slave. Gothi was also learning how to utilize a Mega Stone that she discovered called Gothitelleite. She forced the Mega Evolution guru to teach her to Mega Evolve without the bond of a trainer, something that she successfully did.
The Tower eventually was heavily damaged and almost destroyed when Mega Gothi took on Paige upon the rooftop. Gothi was victorious on this occasion.