The Moon Tower is a room in Greenhaven castle used for helping connect with your spirit animal.
The tower has a jumble of 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 to try to persuade her to find Dinesh's talisman and save Melin's faltering bond with Jhi. Finn tells her he has used it to help his bond with Donn and is using it again. He then convinces Meilin to use it, and as she plays a Zhongese instrument she had been taught to play, Jhi "proves" her use: to show Melin her options and help her clear her mind.