I was just thinking how it would be good to be able to attack an enemy primarch on a bubbled castle.
For example, if a castle bubbles then both the castle, and the primarchs of the team whose castle it is are both protected on the castle and prohibited from hitting on the castle. BUT, anyone else can attack a non-resident primarch over the bubble.
This would allow for a pre-emptive attack on a would be sniper moving in to a castle.
Essentially, bubbles should not be safe zones, they should be red zones for all but the castle owner.