I'm playing my first game and I think I'm doing quite good. I've started as a bohemian duke and I've managed to create Kingdom of Bohemia, expanded a little while HRE had low crown authority, got elected as an Emperor, created a Kingdom of Germany. I lost my Emperor title after death of my first ruler. So now I remain a King of Bohemia and Germany and I managed to expand to Poland a little. The question is - is it possible at all to expand withing the Empire? Crown authority is high at the moment. I can't fight the Emperor's forces now and I can't wage wars again fellow vassals. I can only expand into Poland and/or Hungary - should I do it in hopes I would be able to amass an army large enough to challenge the emperor himself?
When the crown authority prevents you from waging wars with other vassals, there are a few things you can do:
Start a Plot to force your liege to lower crown authority.
You can invite other vassals of your liege to join your plot, although this will depend greatly on how much they like you and how much they like your liege. I often try to grease the wheels by giving them gifts or fulfilling wishes ("wants to marry"). You also could have your chancellor sow distrust, although in my experience that does not succeed often.
Careful manipulation of marriage and murder.
This is normally best done with young adults who are over 16. Find a Duke or Count that has one son (his heir) and a daughter. Arrange a marriage between YOUR heir and THEIR daughter. Once the marriage is nice and locked in, see if you can't knock the only son out of the picture with a well placed assassination plot. It will also be helpful if the target Duke or Count is older in years along with his wife. Keep an eye on his wife from time to time to make sure she is not pregnant.
Sometimes the easiest thing to do is just wait for the current leader to die and the realm to fall into anarchy. Civil war will often break out after an Emperor dies, especially if the heir to the throne is young or if there are many claimants in the family. You can take advantage of the chaos and claim independence, using mercenaries to strike quick blows to force the new liege to yield. This is risky, however, due to the possibility of the Emperor regaining control and bringing about a stable rein. Generally, the more chaos and unhappiness there is within the Empire, the easier it will be to tear pieces away.
Just a few ideas. There are always more ways to get from A to B. I enjoy the challenge of claiming territory within a larger Empire, but you do get a bit stuck when crown laws get to high. In my experience, however, the more they tighten their grip, the more vassals get revolt-happy.
There are a few ways you can expand.