I think I explained my idea to some people a little bit ago but here it is:
So you have a campaign that has five stages; Let's call these 1,2,3,4,5.
Let's call the teams A and B
The campaign starts at 3 which is something like a conquest map.
Then depending on which side wins, the campaign moves to either 2(A) or 4(B) which is a regular linebattle.
Let's say that the A has won.
Then depending on which team wins, the campaign either moves back to the conquest stage(3) if B wins or advances onto the siege phase(1) if A wins.
Then if A attacks the castle(1) and wins a best of 3? they win the campaign. However if they lose, then if returns back to the linebattle(2).