Trace of a Vanished Civilization III

Throughout the Tehmelun Ruins, traces of doors that were used as passages in the past can be found. The common feature is that clear traces of use remain, but they appear to have been blocked in a way that prevents passage. Even places assumed to have been windows are solidly blocked with stones, leading scholars to speculate that these traces may have been measures to prevent something from breaking into the ruins in the past. Who were the invaders of Tehmelun's past that sealed its doors? Or... were they trying to prevent something from escaping from within?