How do I start an online siege?

[$wagvin]

Wheres the option to do it? do I have to unlock it or smth

[captainfriggan]

Should see a castle looking icon (be out in front of your fortresses)

[ghoulhound83]

You have to get to a story mode part of it first in chapter 2...

First you must conquer that region.

Once conquered, an orange icon will show up on your map just right outside your front fort gate...

Go there and good luck.