It's a huge tower in the garden inside the Palace of Shadows. There are 7 rooms(I think) inside and each one has a riddle on the wall, if you figure out the riddle and do what it says you get a key, you need seven keys.
It's near the place in Palace of Shadow where you find Chain Chomps. There's water and you fight Doopliss, Marilyn, 8eldam there. The Tower is the thing in the back. You must go to the Place of Shadow in chapter 8, go into 8 rooms to solve 8 riddles for 8 keys and then put them in the 8 pedestals and then KA-ZOWWWW!! Watch the 8 planets move into alignment and then KA-ZOWWWW!!! A huge EXPLOSION!!!!