The Moon Tower is a room in Greenhaven castle used for helping connect with your spirit animal.
The tower has a jumble of instuments and play instruments, sorted together by their element. Woodwind instruments and wind related things grouped by air, etc.
It is first introduced in Hunted, as Finn shows it to Meilin in order to try to get her to look for the talisman, and to not let her faltering bond with Jhi break. Finn tells her he has used it to help his bond with Donn, and is using it again. He then talks Meilin into using it, and as she plays a Zhongese instrument she had been taught to play, Jhi shows her her options to choose from as planets, and helps her to think.