Tenkai Fortress

The Tenkai Fortress (Japanese: 天界の要塞, Tenkai no Yousaii) is a legendary and mysterious place, located on Quarton. It has been told to have great powers inside of it. The Corrupted and the Corekai are looking for it. Its specific location on Quarton and what it looks like hasn't been revealed yet, however Vilius was looking for it underground.

In the Series
It was first mentioned in "No "I" in Team" by Granox, then by Mr. White, where he was explaining to the boys about it and they must find it before Vilius does.