Tenkai Fortress

The Tenkai Fortess is a legendary and mysterious place, located on Quarton. Its been told to have great powers inside of it. The Corrupted and the Corekai are looking for it. Its specific location and what it looks like hasn't been revealed yet.

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.