Sergeant Allenos:
The Seed of Destruction is essentially a gigantic fortress. Throughout its corridors are barriers and doors controlled by several devices guarded by Mounted Troop soldiers. To penetrate into the interior, you must attack those devices. Even then, you'll need to force open the doors.
When you get inside, you'll see the fortress for yourself. Since this is Tiat's last line of defense, there will be a large number of Mounted Troop soldiers. Defeat them and you'll reach the Throne Room and Tiat herself.
A word of advice: move as quickly as possible through the fortress. The longer it takes you, the more enemies will appear.