Modern Day Earth
The Thousand Worlds are called such because in addition to the Twelve planets, there are other habitable places that were crafted completely by magic, reachable only by transport gates. In one case, the habitable place was so cunningly and thoroughly crafted as to be a near-perfect replica of the solar system, but with only one habitable region. That region is our modern-day Earth.
If the Thousand Worlds can be called “the jailhouse” because of its near-total isolation from the rest of the multiverse, modern-day Earth could be considered solitary confinement or maximum security. There is only one known gate between modern-day Earth and the Thousand Worlds, and only a handful of souls have the power to travel freely between modern-day Earth and the other worlds without using said gate. This makes modern-day Earth a very popular place for the Yggdraverse’s “law enforcement” to relocate evil or destructive entities that threatened lives elsewhere.
Not all the powerful beings found on Earth are evil or destructive, though. Nicole Matthews may tell you she is, but until very recently she suffered from seriously impaired perspective. And she would readily argue that her friends Wes and Lara and Amelia are not evil, though they easily rival or exceed her in power.