I guess you could formalise that with a starting difficulty level selection (e.g. "easy" which goes 1-4; "normal" which goes 2-5; and "hard" which goes 3-6). But it's strictly not necessary since you can just modify the numbers yourself.
You can certainly handicap yourself by adding a difficulty level to the battles. I find level 6 a bit frustrating because your troops are lower quality. But it might be interesting as a Hail Mary-style option for a player - ie, a more affordable rebellion, or a force that's very cheap to raise. Let me think how to do it...
[quoteApologies if this is covered in the manual which I must admit to skim reading very quickly just so I could understand how to get into play and do the first turn or so. [/quote]
Not at all! Even the best manuals are hard to read and are usually best as reference once you know the game - and the ToC manual still needs a lot of tightening.
From your comments about playing with low casualties at levels do I take it that victory in meeting the normal battle criteria (ie 40/60%) will be translated into different victory outcomes in campaign through the variable rout level which gets harder (ie lower for enemy losses), depending on success as the game progresses?
That's roughly correct. Winning battles increases the odds that the enemy is willing to face you at - the difficulty level, in game terms. There are often one or more pre-battle phases where the enemy can choose to improve the odds. But they risk losing the province altogether without a fight while doing so - so it's not a viable strategy for a player.
It's a bit like the AI are the Persians facing Alexander. After Granicus, and even more so after Issus, the regional satraps are really reluctant to fight. But while they stay out of reach, Egypt and half of the western satrapies all capitulate without a fight. Only when Darius gets together a really big army, at Gaugamela, is there a battle. The intention is that this doesn't feel like a gamey strategy to the player, because no player would willingly choose to give up half the empire like that.
As for battle losses, the player needs a certain threshhold (35%) to win against an enemy army. Otherwise it's a draw. Against a provincial garrison, just a simple victory is fine. Higher losses however reduce the player commander's "vitality" and can cut short his career. This basically represents wounds, the commander's sense that his "luck" is running out, etc.
There's no hard and fast rule though for what constitutes reasonable losses in a battle, other than the 35% rule for fighting an an enemy army. Just more losses = a shorter career.
Does that make sense? I hope it provides the right amount of escalating challenge dynamically. I've thought of some other more complicated ways to combine 1) loss threshhold 2) the difficulty rating, 3) the amount of resources a player wishes to commit to an attack on a province, but this one I hope is fairly simple.