If I'm not wrong, you need to be assigned a Duchy in order to secede & form a new realm now. You can't just take a city & break away.
This is correct. Only a Duke has the option to secede. The duke has to have a duchy that has a city/stronghold, and it *cannot* contain the capital city/stronghold.
So - you can't start a new realm, unless you're already friends with a Ruler.
Or you know how to make friends with a ruler. Try suggesting credible ways that you can carve your new realm out of his enemy's lands. That usually works much better than convincing them to give up their own lands to make a new realm.
