First Mission Horde Undead Campaign - Chapter I

[PaLhAçO GoZO]

One of the primary objectives is:

"Obtain the Lord's seal for Murdoch Mourncliff"

And at the bottom of the map there are two bridges with a bubble (like a protective field) forbidding my passage to attack the empire. I already explored the entire map and I do not know what else to do, did some kind of bug happen? Please someone help me !!