You will be warped to the bastion directly after you kill the chapter Nidhoggr's Revenge.
Getting to Bastion
Start of Eden Dungeon [Lv:3]. The portal to Bastion is marked as a yellow star '★' on the map.
This is the portal to Bastion:
Getting to Eclage Dungeon
After entering the portal, you will be teleported to Bastion. If you walk to the right, you can click on a Soldier who appears 'hidden' among the crystals. Talk to him to have him warp you to Eclage Dungeon.
If you've already submitted Lost Soul, you will be warped to Eclage Dungeon [Lv:2] instead.