User blog comment:Observer Supreme/Who built the Tower of Heaven?/@comment-25324252-20160124224730/@comment-7418318-20160124225621

To elaborate a bit: There was a really big cult that attempted to build several Towers, though it was never explained why they needed more than one.

The Magic Council found out and wiped out most of them, including all but one tower.

The last tower, situated on a remote island, wasn't found and that's where the last remnants of the cult survived. The set sail to hunt for children to use as slave labour and that brings us right where the article that Gisme linked starts off.