When you get to the Astreas Bastion, you will be asked for a Village gate key in order to acess one of the routes leaving the town. To recieve the gate key you must beat Petunia The Angry, the fourth boss, and return to the nearest sigil in order to comple the quest Anger Just Leads To More Violence, which upon completion will reward you with the village gate key. If you head back to the Petunia The Angry arena and go just beyond that to the forge you will discover that you can open a gate using the village key from there, giving you easy acess to Astreas Bastion.